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Photometric and spectroscopic analysis of YY CrB
The eclipsing binary YY Coronae Borealis is a short-period contactsystem of the W UMa type (sub-type A) and one of the systems with agreat potential for providing a combined photometric and spectroscopicsolution. Using WD and FOTEL codes we reanalyzed the system, and applieda spotted model to treat the asymmetry of the light curve. The resultsshow that the more massive component is cooler than the less massivewith ΔT˜100K. The system is an overcontact one with fill-outfactor approximately equal to 64% and distance to the system 88±7pc.
| The Minimum Mass Ratio for Contact Close Binary Systems of W Ursae Majoris-type
Extreme mass ratio close binaries of W UMa-type represent an interestingclass of objects in which ˜1M main-sequence star is in contactwith a significantly less massive companion (M˜0.1M). Earliertheoretical investigations of these systems found that there is aminimum mass ratio q=M/M≈0.085-0.095 (obtained for n=3polytrope-fully radiative primary) above which these systems are stable.If the mass ratio is lower than minimum, a tidal instability develops(Darwin's instability). This instability, which is secular, growing on aviscous dissipation timescale, eventually forces the stars to merge intoa single, rapidly rotating object (such as FK Com-type stars or bluestragglers), implying that such systems would not be observed. Thereappear to be, however, some W UMa-type binaries with empiricallyobtained q values below the theoretical limit for stability. The aim ofthis dissertation was to try to resolve the discrepancy between theoryand observations by considering rotating polytropes. Other candidatesystems for stellar mergers such as AM CVn-type stars have also beendiscussed in the dissertation.
| An Orbital Period Investigation of the Solar-Type Overcontact Binary V700 Cygni
Not Available
| Possible solution to the problem of the extreme mass ratio W UMa-type binaries
When the total angular momentum of a binary system is at a critical(minimum) value, a tidal instability occurs (Darwin's instability),eventually forcing the stars to merge into a single, rapidly rotatingobject. The instability sets in at some critical separation which in thecase of contact binaries corresponds to a minimum mass ratio dependingon dimensionless gyration radius k1. If one considers n = 3polytrope (fully radiative primary with Γ1 = 4/3),k21 = 0.075 and qmin ~ 0.085-0.095.There appears to be, however, some W UMa-type binaries with q valuesvery close, if not below these theoretical limits, implying that primaryin these systems is probably more centrally condensed. We try to solvethe discrepancy between theory and observations by considering rotatingpolytropes. We show by deriving and solving a modified Lane-Emdenequation for n = 3 polytrope that including the effects of rotation doesincrease the central concentration and could reduce qmin toas low as 0.070-0.074, more consistent with the observed population.
| On Period Variations of the K-Type Eclipsing Binary BM Ursae Majoris
From new CCD observations, 11 light minimum times are presented in thispaper. The orbital period change of a W UMa-type eclipsing binary, BMUrase Majoris, was investigated, based on all available times of lightminimum. It was discovered that the orbital period of the binary showscyclic oscillation, superimposed on a secular period decrease. Theamplitude and the period of the cyclic variation are A =0.0101(±0.0004)d and P3 = 30.8(±0.2)yr,respectively. This kind of cyclic variation may be due to a light-timeeffect via an unseen third body. If the existence of an additional bodyis true, it may remove a great amount of angular momentum from thecentral system, and may play an important role in the formation of closebinary. The orbital period continuously decreases at a rate of dP/dt =‑7.49(±0.16) × 10-8d yr-1,which may be due to mass transfer from the primary to the secondary,accompanying mass and angular-momentum loss from the binary system. Theorbital period decrease, accompanying mass and angular momentum loss,would cause the contact degree to increase. Therefore, this binary willevolve into a deep contact configuration.
| Photoelectric minima of selected eclipsing binaries in 2008.
Not Available
| A New Photometric Investigation of the W UMa-Type Binary BI CVn
New photometric observations and their investigation of the W UMa-typebinary, BI CVn, are presented. The variations of the orbital period wereanalyzed based on 12 new determined times of light minimum together withthe others compiled from the literature. It is discovered that theperiod of BI CVn shows a long-term period decrease at a rate of\dot{P}=-1.51(\pm 0.12)\times {10^{-7}} daysyear-1 while it undergoes a cyclic variation with aperiod of 27.0 years and an amplitude of 0fd0151. Photometricsolutions determined with the Wilson-Devinney method suggest that BI CVnis a contact binary with a degree of contact of 18.0(±1.7)%. Theasymmetry of the light curves was interpreted by the presence of darkspots on both components, and absolute parameters were determined bycombining the photometric elements with the spectroscopic solutionsgiven by Lu. The observed period decrease can be plausibly explained bya combination of the mass transfer from the primary to the secondary andangular momentum loss via magnetic braking. The cyclic periodoscillation suggests that BI CVn is a triple system containing atertiary component with a mass no less than 0.58 M sun in a27.0 year orbit. As in the cases of the other contact binaries (e.g., AHCnc, AP Leo, AD Cnc, and UX Eri), it is possible that this tertiarycompanion played an important role for the formation and evolution ofthe contact system by removing angular momentum from the central systemvia Kozai oscillation or a combination of Kozai cycle and tidalfriction, which causes the eclipsing pair to have a short initialorbital period (e.g., P < 5d). In that case, can theinitially detached system evolve into the present contact configurationvia a combination of magnetic torques from stellar winds and a case Amass transfer?
| Angular momentum and mass evolution of contact binaries
Various scenarios of contact binary evolution have been proposed in thepast, giving hints of (sometimes contradictory) evolutionary sequencesconnecting A- and W-type systems. As the components of close detachedbinaries approach each other and contact binaries are formed, followingevolutionary paths transforms them into systems of two categories:A-type and W-type. The systems evolve in a similar way but underslightly different circumstances. The mass/energy transfer rate isdifferent, leading to quite different evolutionary results. Analternative scenario of evolution in contact is presented and discussed,based on the observational data of over one hundred low-temperaturecontact binaries. It results from the observed correlations amongcontact binary physical and orbital parameters. Theoretical tracks arecomputed assuming angular momentum loss from a system via stellar wind,accompanied by mass transfer from an advanced evolutionary secondary tothe main-sequence primary. A good agreement is seen between the tracksand the observed graphs. Independently of details of the evolution incontact and a relation between A- and W-type systems, the ultimate fateof contact binaries involves the coalescence of both components into asingle fast rotating star.
| High Fill-Out, Extreme Mass Ratio Overcontact Binary Systems. VIII. EM Piscium
CCD photometric observations of the newly discovered close binary, EMPiscium, obtained from 2006 December 4 to 2008 January 7, are presented.The light curves are symmetric and show complete eclipses with aneclipse duration of 54 minutes. When comparing the present light curveswith those published by González-Rojas et al., it is found thatthe depths of the two minima of the light curve have been interchanged,and the positive O'Connell effect has disappeared. The symmetric lightcurves in R and I bands were analyzed with the 2003 version of the W-Dcode. It is found that EM Piscium is a high fill-out overcontact binarysystem (f = 95.3 ± 2.7%) with an extreme mass ratio of q =0.1487, suggesting that it is on the late evolutionary stage oflate-type tidal-locked binaries. Based on the nine instances of lightminimum that we determined and those published by previousinvestigators, it is discovered that the orbital period shows a cyclicperiod variation with a period of 3.3 years, while it undergoes acontinuously rapid increase at a rate of dP/dt = +3.97 ×10-6 days year-1. The cyclic periodreveals the presence of a tertiary companion, which may play animportant role for the formation and evolution of the overcontact binaryby drawing angular momentum from the central system via Kozaioscillation or a combination of Kozai cycle and tidal friction. The highfill-out, the extreme mass ratio, and the rapid period increase maysuggest that the binary system is quickly evolving into a rapid-rotatingsingle star.
| DN Bootis: A low mass-ratio W UMa-type contact binary
New photoelectric BVR light curves and radial velocity curves wereobtained for the HIPPARCOS discovery DN Boo at theTÜBİTAKTÜBİTAK: The Scientific and TechnicalResearch Council of Turkey.1 National Observatory of Turkey(TUG) and Dominion Astrophysical Observatory (DAO), respectively, todetermine physical nature of the variable. The character of the obtainedlight curves and double-lined spectroscopic structures in the obtainedspectra are revealed that DN Boo is a genuine EW type eclipsing binary.During the analysis of our new observations a simultaneous solution werederived for the photometric and spectroscopic data by using the WilsonDevinney code and orbital parameters with absolute dimensions of thesystem were determined for the first time. Finally, the importance ofvery low mass-ratio contact binaries in the late stages of close binaryevolution was discussed.
| V802 Aquilae: A Short-Period Eclipsing Binary with Magnetic Activity
New CCD observations for V802 Aql were observed from 2004 to 2007, andseven light minimum timings were obtained. Using the Wilson-Devinneycode, the photometric solution was deduced from R-band observations. Theresults show that the mass-ratio and the contact degree are q =0.1443±0.0059 and f = 34.9%±7.6%, respectively. Comparingthe parameters of the dark spot from our results with those from Samecet al., it is found that the magnetic activity of the primary componentmay have been migrating towards its equator from 2002 to 2005. Anorbital period analysis weakly suggests that the orbital period mayundergo a cyclic variation with a period of P3 = 8.32yr andan amplitude of A = 0d.0196. The periodic oscillation of thisbinary may be more plausibly explained as being the result of magneticactivity cycles. Therefore, the eclipsing star V802 Aql may becoalescing via magnetic breaking.
| CCD Photometric Study of the W UMa-Type Binary II Canis Majoris in the Field of Berkeley 33
The CCD photometric data of an EW-type binary, IICMa, which is a contactstar in the field of the middle-aged open cluster Berkeley33, arepresented. Acomplete R light curve was obtained. In the present work,using the five CCD epochs of light minimum [three of them werecalculated from Mazur etal. (1993 MNRAS, 265, 405)'s data and two fromour new data], the orbital period P was revised to 0.22919704days. Thecomplete R light curve was analyzed by using the 2003 version of W-D(Wilson-Devinney) program. It is found that this is a contact systemwith a mass ratio of q=0.9 and a contact factor of f=4.1%. The high massratio (q=0.9) and the low contact factor (f=4.1%) indicate that thesystem just evolved into the marginal contact stage.
| The evolutionary status of W Ursae Majoris-type systems
Well-determined physical parameters of 130 W Ursae Majoris (W UMa)systems were collected from the literature. Based on these data, theevolutionary status and dynamical evolution of W UMa systems areinvestigated. It is found that there is no evolutionary differencebetween W- and A-type systems in the M-J diagram, which is consistentwith the results derived from the analysis of observed spectral type andof M-R and M-L diagrams of W UMa systems. M-R and M-L diagrams of W- andA-type systems indicate that a large amount of energy should betransferred from the more massive to the less massive component, so thatthey are not in thermal equilibrium and undergo thermal relaxationoscillation. Moreover, the distribution of angular momentum, togetherwith the distribution of the mass ratio, suggests that the mass ratio ofthe observed W UMa systems decreases with decreasing total mass. Thiscould be the result of the dynamical evolution of W UMa systems, whichsuffer angular momentum loss and mass loss as a result of the magneticstellar wind. Consequently, the tidal instability forces these systemstowards lower q values and finally to rapidly rotating single stars.
| AD Cancri: A Shallow Contact Solar-Type Eclipsing Binary and Evidence for a Dwarf Third Component and a 16 Year Magnetic Cycle
CCD photometric observations of AD Cancri obtained from 2000 March 7 to2004 December 20 are presented. Variations of the light levels at theprimary minimum and both maxima are found. Uniform solutions of foursets of photometric data were derived by using the Wilson-Devinneymethod. The solutions suggest that AD Cancri is a shallow W-type contactbinary (f=8.3%+/-1.3%) with a high mass ratio of 1/q=0.770+/-0.002. Thelong-term variation of the light curve is explained by variabledark-spot models of the more massive component star with a possible 17yr cycle. Our 13 times of light minimum over 5 years, including otherscollected from the literature, have been used for the period study. Thecomplex period changes can be sorted into a long-term period increase atrate of dP/dt=+(4.94+/-0.16)×10-7 days yr-1,a 16.2 yr periodic component (A3=0.0155 days), and a verysmall amplitude period oscillation (A4=0.0051 days,P4=6.6 yr). The existence of third light may indicate thatthere is a tertiary component in the binary system. Solving thefour-band light curves of Samec & Bookmyer, it is found that thecontribution of the tertiary component to the total light of the triplesystem increases with wavelength, which suggests that it is very cooland may be a very red main-sequence star. The small-amplitude periodoscillation may be caused by the light-time effect of the cool tertiarycomponent (M3~0.41 Msolar). The 16.2 yr periodiccomponent in the orbital period and the 17 yr cyclic activity of thedark spot on the more massive component both may reveal that the moremassive component displays solar-type magnetic activity with a cyclelength of about 16 yr.
| Contact Binaries with Additional Components. III. A Search Using Adaptive Optics
We present results of the Canada-France-Hawaii Telescope adaptive optics(AO) search for companions of a homogeneous group of contact binarystars, as a contribution to our attempts to prove the hypothesis thatthese binaries require a third star to become as close as observed. Inaddition to directly discovering companions at separations of>=1″, we introduced a new method of AO image analysis utilizingdistortions of the AO diffraction ring pattern at separations of0.07″-1″. Very close companions, with separations in thelatter range, were discovered in the systems HV Aqr, OO Aql, CK Boo, XYLeo, BE Scl, and RZ Tau. More distant companions were detected in V402Aur, AO Cam, and V2082 Cyg. Our results provide a contribution to themounting evidence that the presence of close companions is a very commonphenomenon for very close binaries with orbital periods <1 day.Based on observations obtained at the Canada-France-Hawaii Telescope,which is operated by the National Research Council of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientifique of France, and the University of Hawaii.
| A Mechanism for Orbital Period Modulation and Irregular Orbital Period Variations in Close Binaries
Orbital period modulation is observed in many magnetically active closebinaries. It can be explained by magnetic connection between two stars.Magnetic connection produces weak force between the two stars. As themagnetic field varies periodically, the orbital period also showscyclical variations. The mechanism can also be used to explain irregularorbital period variations and orbital period jumps. The mean surfacemagnetic strength is calculated by using the Radia package, which isdedicated to 3D magnetostatics computation. On the basis of the results,a practical equation is given to calculate surface magnetic strength.
| Variation of the period and light curves of the solar-type contact binary EQ Tauri
We present two new sets of complete light curves of EQ Tauri (EQ Tau)observed in 2000 October and 2004 December. These were analysed,together with the light curves obtained by Yang & Liu in 2001December, with the 2003 version of the Wilson-Devinney code. In thethree observing seasons, the light curves show a noticeable variation inthe time-scale of years. The more massive component of EQ Tau is asolar-type star (G2) with a very deep convective envelope, which rotatesabout 80 times as fast as the Sun. Therefore, the change can beexplained by dark-spot activity on the common convective envelope. Theassumed unperturbed part of the light curve and the radial velocitiespublished by Rucinski et al. were used to determine the basic parametersof the system, which were kept fixed for spot modelling in the threesets of light curves. The results reveal that the total spotted area onthe more massive component covers 18, 3 and 20 per cent of thephotospheric surface in the three observing seasons, respectively. Polarspots and high-latitude spots are found. The analysis of the orbitalperiod has demonstrated that it undergoes cyclical oscillation, which isdue to either a tertiary component or periodic magnetic activity in themore massive component.
| Deep, Low Mass Ratio Overcontact Binary Systems. VII. QX Andromedae in the Intermediate-Age Open Cluster NGC 752
QX Andromedae is a short-period eclipsing binary in the intermediate-ageopen cluster NGC 752. Charge-coupled device photometric observations ofthe close binary system obtained from 2004 November 13 to 2006 November18 are presented. It is confirmed that the light curves show partialeclipses, and night-to-night intrinsic variations are seen. As in thecase of AH Cancri in the old open cluster M67, the light curve of QX Andseems to change between A and W types. Both the short- and long-termlight variations suggest that QX And shows strong magnetic activity,which is in agreement with its X-ray observations. The symmetric lightcurves in B and V bands obtained the night of 2004 November 13 wereanalyzed with the new version of the W-D code. It is found that QX Andis a deep overcontact binary system with a high degree of overcontact off = 55.9% and a low mass ratio of q = 0.2327, suggesting that it is inthe late stage of overcontact evolution. Based on our 23 times of lightminimum, including four recently published eclipse times, the orbitalperiod of the eclipsing binary was revised. It was discovered that theorbital period shows a continuous period increase at a rate of dP/dt =+2.48 × 10-7 days yr-1, which can beinterpreted as a mass transfer from the less massive component to themore massive one. As the period increases, the mass ratio of the systemwill decrease. It may finally evolve into a rapid-rotating single starwhen it meets the more familiar criterion that the orbital angularmomentum is less than 3 times the total spin angular momentum. Theexistence of QX And in the late evolutionary stage of an overcontactbinary in the intermediate-age open cluster NGC 752 indicates that itmay undergo strong cluster stellar interaction. This means it had a veryshort initial orbital period and could have evolved into the presentevolutionary state within the cluster age. By comparing with theevolutionary state of TX Cnc in M44, it is estimated that the lifetimeof overcontact binaries may be no less than 1 Gyr.
| Minima Times of Some Eclipsing Binary Stars
We present 48 pe and 14 CCD times of minima of selected eclipsingbinaries obtained from 1989 to 2007 at observatories in Turkey.
| The minimum mass ratio of W UMa-type binary systems
When the total angular momentum of a binary system Jtot =Jorb + Jspin is at a certain critical (minimum)value, a tidal instability occurs which eventually forces the stars tomerge into a single, rapidly rotating object. The instability occurswhen Jorb = 3Jspin, which in the case of contactbinaries corresponds to a minimum mass ratio qmin ~0.071-0.078. The minimum mass ratio is obtained under the assumptionthat stellar radii are fixed and independent. This is not the case withcontact binaries where, according to the Roche model, we haveR2 = R2(R1, a, q). By finding a newcriterion for contact binaries, which arises from dJtot = 0,and assuming k21 ≠ k22for the component's dimensionless gyration radii, a theoretical lowerlimit qmin = 0.094-0.109 for overcontact degree f = 0-1 isobtained.
| A New CCD Photometric Investigation of the Short-Period Close Binary AP Leonis
New CCD photometric light curves in the B, V, and R bands of theshort-period close binary AP Leonis are presented. A photometricanalysis with our symmetric light curves suggests that AP Leo is anovercontact binary with a degree of overcontact of 24.9%. Since the O-Cvalues of photographic and visual times of light minimum showed a largescatter (up to 0.06 days), all of the period changes proposed for theeclipsing binary by previous investigators are not reliable. In thispaper the orbital period changes of AP Leo are analyzed based on allpublished CCD and photoelectric eclipse times. A small-amplitude cyclicoscillation, with a period of 22.4 yr and an amplitude of 0.0049 days,is discovered to be superposed on a secular period decrease at a rate ofdP/dt=-1.08×10-7 days yr-1. The continuousperiod decrease may be caused by angular momentum loss or a combinationof the mass transfer from the primary to the secondary and angularmomentum loss. The cyclic period change may indicate that AP Leo is atriple system containing a cool dwarf third component. If this is true,it is possible that this third component plays an important role in theorigin and evolution of the overcontact system by removing angularmomentum from the central system, and that it makes the eclipsing pairhave a low angular momentum and a short initial orbital period (e.g.,P<5 days). In that case, the initially detached system evolves intoan overcontact configuration via magnetic torques from stellar winds. Onthe other hand, the rapid rotation of the solar-type components(spectral type G0) and the variations of the light curve indicate a highdegree of magnetic activity from the spin-up of the components. Both thelong-term period decrease and the oscillation can plausibly beinterpreted by magnetic activity (i.e., enhanced magnetic stellar windand activity cycles).
| Dynamical evolution of active detached binaries on the logJo-logM diagram and contact binary formation
Orbital angular momentum (OAM, Jo), systemic mass (M) andorbital period (P) distributions of chromospherically active binaries(CAB) and W Ursae Majoris (W UMa) systems were investigated. Thediagrams of and logJo-logM were formed from 119 CAB and 102 WUMa stars. The logJo-logM diagram is found to be mostmeaningful in demonstrating dynamical evolution of binary star orbits. Aslightly curved borderline (contact border) separating the detached andthe contact systems was discovered on the logJo-logM diagram.Since the orbital size (a) and period (P) of binaries are determined bytheir current Jo, M and mass ratio, q, the rates of OAM loss(dlogJo/dt) and mass loss (dlogM/dt) are primary parametersto determine the direction and the speed of the dynamical evolution. Adetached system becomes a contact system if its own dynamical evolutionenables it to pass the contact border on the logJo-logMdiagram. The evolution of q for a mass-losing detached system is unknownunless the mass-loss rate for each component is known. Assuming q isconstant in the first approximation and using the mean decreasing ratesof Jo and M from the kinematical ages of CAB stars, it hasbeen predicted that 11, 23 and 39 per cent of current CAB stars wouldtransform to W UMa systems if their nuclear evolution permits them tolive 2, 4 and 6 Gyr, respectively.
| Orbital period investigations of two short-period early-type overcontact binaries BH Cen and V701 Sco in two extremely young galactic clusters IC 2944 and NGC 6383
Both V701 Sco and BH Cen are two early-type short-period overcontactsystems (P = 0.d762 and P = 0.d792, respectively).V701 Sco is a member of the young galactic cluster NGC 6383, while BHCen is a component of a younger galactic cluster IC 2944 where starformation is in process. They provide good opportunity to understand theformation and evolution of binary stars. In the present paper, orbitalperiod changes of the two binaries are investigated. It is discoveredthat the orbital period of BH Cen shows a long-term increase with a rateof dP/dt = +1.70(±0.39) × 10‑7 days/yearwhile it undergoes a cyclic oscillation with a period of 44.6 years andan amplitude of A3 = 0.d0216. For V701 Sco, itsO-C curve reveals a periodic change with a period of 41.2 years andamplitude of A3 = 0.d0158. The mass ratio of BHCen is 0.84, but V701 Sco contains twin B1-1.5V type stars with a massratio of unit. The continuous period increase of BH Cen is caused by themass transfer from the less massive component to the more massive one ata rate of dM2/dt = 3.5 × 10‑6days/year. The cyclic period changes of both systems can be plausiblyexplained as the results of light-travel time effects suggesting thatthey are triple systems. The astrophysical parameters of the unseentertiary components in the two systems have been determined. We thinkthat the invisible tertiary components in both binaries played animportant role in the formations and evolutions of the overcontactconfigurations by bringing angular momentum out from the centralsystems. For BH Cen, this process created the initial short period andwill support its evolution into an overcontact configuration via a CaseA mass transfer within the life time of the extremely young cluster IC2944. For V701 Sco, two identical zero-age main-sequence components inan overcontact configuration suggest that it may have been formed byfission, possibly by the fission of the third body. The fact that nolong-term continuous period variations were found for V701 Sco maysuggest that an overcontact binary with the mass ratio of unity can bein an equilibrium revealing that the original configuration of thebinary was overcontact as is its present state. It has been reportedthat faint stars in the two extremely young clusters are relativelyscare. From the present study, it is shown that faint stars in youngclusters are usually formed as companions of OB stars (includingbinaries). It is very difficult to detect them because of their lowluminosity when compared with the more luminous OB stars.
| Contact Binaries with Additional Components. II. A Spectroscopic Search for Faint Tertiaries
It is unclear how very close binary stars form, given that during thepre-main-sequence phase the component stars would have been inside eachother. One hypothesis is that they formed farther apart but were broughtin closer after formation by gravitational interaction with a thirdmember of the system. If so, all close binaries should be members oftriple (or higher order) systems. As a test of this prediction, wepresent a search for the signature of third components in archivalspectra of close binaries. In our sample of 75 objects, 23 show evidencefor the presence of a third component, down to a detection limit oftertiary flux contributions of about 0.8% at 5200 Å (consideringonly contact and semidetached binaries, we find 20 out of 66). In ahomogeneous subset of 59 contact binaries, we are fairly confident thatthe 15 tertiaries we have detected are all tertiaries present with massratios 0.28<~M3/M12<~0.75 and implied outerperiods P<~106 days. We find that if the frequency oftertiaries were the same as that of binary companions to solar-typestars, one would expect to detect about 12 tertiaries. In contrast, ifall contact binaries were in triple systems, one would expect about 20.Thus, our results are not conclusive but are sufficiently suggestive towarrant further studies.
| Masses and angular momenta of contact binary stars
Results are presented on component masses and system angular momenta forover 100 low-temperature contact binaries. It is found that thesecondary components in close binary systems are very similar in mass.Our observational evidence strongly supports the argument that theevolutionary process goes from near-contact binaries to A-type contactbinaries, without any need of mass loss from the system. Furthermore,the evolutionary direction of A-type into W-type systems with asimultaneous mass and angular momentum loss is also discussed. Theopposite direction of evolution seems to be unlikely, since it requiresan increase of the total mass and the angular momentum of the system.
| On the Period Variation of the W UMa-type Contact Binary V502 Ophiuchi
The variation in the orbital period of the W UMa type contact binaryV502 Oph is analyzed. The orbital period exhibits a wavelike variationwith a periodicity of 23.0 years and an amplitude of ΔP =1.24×10-6 days superimposed on secular decrease ofdP/dt = 1.68×10-7 day per year. The long-term decreasemay be accompanied by the contraction of the secondary at a rate of 83 mper year and a mass transfer rate from the primary to the secondary of4.28×10-8Modot per year. The short-termoscillation may be explained by the presence of a third component.Orbital elements of the third body and its possible mass are presented.
| Deep, Low Mass Ratio Overcontact Binary Systems. VI. AH Cancri in the Old Open Cluster M67
CCD photometric light curves in the B and V bands obtained in 2001 andin the V band obtained in 2002 of AH Cnc in the old open cluster M67 arepresented. It is shown that AH Cnc is a total-eclipsing binary and itslight curves correspond to a typical A type according to Binnendijk'sclassification. The variations of the light curve around the primaryminimum and second maximum were found. Our nine epochs of light minimummonitored from 2001 to 2005, including others collected from theliterature, were used to create the first study of the period changes ofthe binary system. A cyclic oscillation with a period of 36.5 yr and anamplitude of 0.0237 days was discovered to be superposed on a continuousperiod increase (dP/dt=3.99×10-7 days yr-1).Weak evidence indicates that there exists another small-amplitude periodoscillation (A4=0.0035 days, P4=7.75 yr). Thesymmetric light curves in the B and V bands obtained in 2001 wereanalyzed with the 2003 version of the Wilson-Devinney code. It isconfirmed that AH Cnc is a deep overcontact binary system with a highdegree of overcontact f=58.5%+/-4.5% and a low mass ratio ofq=0.1682+/-0.0012. The existence of the third light and the cyclicperiod oscillation both may suggest that AH Cnc is a triple systemcontaining an unseen third body. The tertiary component may have playedan important role in the origin of the overcontact binary star byremoving angular momentum from the central system, which would cause itto have a short initial orbital period and thus evolve into anovercontact configuration by angular momentum loss. The long-term periodincrease can be interpreted as a mass transfer from the less massivecomponent to the more massive one. As the orbital period increases, thedecrease of the mass ratio will cause it finally to evolve into a singlerapid-rotating star when the system meets the more familiar criterionthat the orbital angular momentum be less than 3 times the total spinangular momentum. Therefore, AH Cnc may be a progenitor of the bluestraggler stars in M67.
| Contact Binaries with Additional Components. I. The Extant Data
We have attempted to establish observational evidence for the presenceof distant companions that may have acquired and/or absorbed angularmomentum during the evolution of multiple systems, thus facilitating orenabling the formation of contact binaries. In this preliminaryinvestigation we use several techniques (some of themdistance-independent) and mostly disregard the detection biases ofindividual techniques in an attempt to establish a lower limit to thefrequency of triple systems. While the whole sample of 151 contactbinary stars brighter than Vmax=10 mag gives a firm lowerlimit of 42%+/-5%, the corresponding number for the much better observednorthern-sky subsample is 59%+/-8%. These estimates indicate that mostcontact binary stars exist in multiple systems.
| BO Canum Venaticorum and SS Comae Berenices: A Photometric Study of AW UMa-Type Binaries
BO CVn and SS Com are two short-period W UMa-type binary stars withspectral types earlier than F5. In the present paper new CCD photometriclight curves in B, V, and R bands of BO CVn and the first complete lightcurve in V band of SS Com are presented. The light curves of the twobinaries are symmetric, and no variations of the light curves were foundfor both systems that are quite unlike those of cooler overcontactbinaries. These observational properties may suggest weak photosphericdark spot activity in the two systems during the observational timeintervals. Photometric solutions of the two W UMa-type binaries werederived by using the 2003 version of the Wilson-Devinney (W-D) method.The new solutions suggest that they are overcontact binary stars withdegrees of overcontact of f=40.6% for BO CVn and f=49.6% for SS Com. Ournew times of light minimum confirmed the long-time period increases ofthe two systems, and the rates of continuous period increase wererevised. The long-time period increases, the orbital periods, thedegrees of overcontact, and the mass ratios all suggest that they are onan evolutionary stage of mass transfer from the less massive componentto the more massive one, and SS Com will reach the present evolutionarystate of BO CVn. The slow evolution toward extreme mass ratio will causethem finally to coalesce to single stars. Therefore, both of them are AWUMa-type near-coalescent overcontact binary systems.
| A catalogue of eclipsing variables
A new catalogue of 6330 eclipsing variable stars is presented. Thecatalogue was developed from the General Catalogue of Variable Stars(GCVS) and its textual remarks by including recently publishedinformation about classification of 843 systems and making correspondingcorrections of GCVS data. The catalogue1 represents thelargest list of eclipsing binaries classified from observations.
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