2005AJ....129.2400H

We present K-band spectra for 12 cataclysmic variables (CVs) with orbital periods under 6 hr. We confidently detect the secondary stars in nine of these systems and may have detected them in the other three. Nine of the 12 CVs clearly have CO first-overtone absorption features that are weaker than they should be for the derived spectral type. We demonstrate that, in general, the weak CO features are due to a carbon deficiency in the secondary star. In the case of U Gem, UU Aql, and TW Vir the carbon abundance in the secondary star appears to be very low, likely only a few percent of the solar value. Deficits of carbon, when combined with the detection of ¹³CO and the ultraviolet detections of enhanced levels of nitrogen in other CV systems, imply that material that has been processed through the CNO cycle is finding its way into the photospheres of CV secondary stars. While several plausible models exist to explain unusual levels of CNO species in CV secondary stars, they do not detail how such species as aluminum, magnesium, or silicon (elements that show abundance anomalies in our spectra) will behave. It appears that the standard model for the formation and evolution of CVs needs substantial revision.