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K-band spectroscopy of pre-cataclysmic variables
Aims. There exists now substantial evidence for abundance anomalies in anumber of cataclysmic variables (CVs), indicating that the photosphereof the secondary star incorporates thermonuclear processed material.However, the spectral energy distribution in CVs is usually dominated bythe radiation produced by the accretion process, severely hindering aninvestigation of the stellar components. On the other hand, depending onhow the secondary star has acquired such material, the above mentionedabundance anomalies could also be present in pre-CVs, i.e. detachedwhite/red dwarf binaries that will eventually evolve into CVs, but havenot yet started mass transfer, and therefore allow for an unobstructedview on the secondary star at infrared wavelengths. Methods: We havetaken K-band spectroscopy of a sample of 13 pre-CVs in order to examinethem for anomalous chemical abundances. In particular, we study thestrength of the 12CO and 13CO absorption bandsthat have been found diminished and enhanced, respectively, in similarstudies of CVs. Results: All our systems show CO abundances that arewithin the range observed for single stars. The weakest 12CObands with respect to the spectral type are found in the pre-CV BPM71214, although on a much smaller scale than observed in CVs.Furthermore there is no evidence for enhanced 13CO. Takinginto account that our sample is subject to the present observationalbias that favours the discovery of young pre-CVs with secondary stars oflate spectral types, we can conclude the following: 1) our studyprovides observational proof that the CO anomalies discovered in certainCVs are not due to any material acquired during the common envelopephase, and 2) if the CO anomalies in certain CVs are not due toaccretion of processed material during nova outburst, then theprogenitors of these CVs are of a significantly different type than thecurrently known sample of pre-CVs.Based on observations made at ESO-Paranal, proposals 075.D-0012 and076.D-0538.
| The Spacelab-1 Very Wide Field Survey of UV-excess objects. IV. The performance of the instrument in combination with optical photometry as a means of identifying stars with peculiar properties.
UV (195 nm) and Stroemgren uvby photometry of a 110 square degree fieldat high southern galactic latitudes are analyzed through a comparison ofi) UV magnitudes for 57 stars of various types common to the publishedTD1 catalogue and the Very Wide Field Camera (VWFC); and ii) observedand theoretical two-colour diagrams. The higher sensitivity of the VWFC(=~0.5magnitude) and its more complete survey are exemplified by thedetection and UV measurement of a series of objects with moderateUV-excess in addition to detection of some very blue objects of variousnature down to fainter than 12th magnitude in the optical domain. Adeeper survey with a VWFC-type instrument could provide a completesample for studies of the group properties of faint blue stars. Duringthe uvby reductions it was found that the usual procedure of plottingresiduals as functions of declination, hour angle and airmass can be apowerful and diagnostic test of photometer rigidity.
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