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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.
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