K-Band Spectroscopy of (Pre-)Cataclysmic Variables: Are Some Donor Stars Really Carbon Poor?

We present a new sample of K-band spectral observations for cataclysmic variables (CVs): non-magnetic and magnetic as well as present-day and pre-CVs. The purpose of this diverse sample is to address the recent claim that the secondary stars in dwarf novae are carbon deficient, having become so through a far more evolved evolution than the current paradigm predicts. Our new observations, along with previous literature results, span a wide range of orbital period and CV type. In general, dwarf novae in which the secondary star is seen show weak to no CO absorption while polar and pre-CV donor stars appear to have normal CO absorption for their spectral type. However, this is not universal. The presence of normal looking CO absorption in the dwarf nova SS Aur and the hibernating CV QS Vir and a complete lack of CO absorption in the long-period polar V1309 Ori cloud the issue. A summary of the literature pointing to non-solar abundances including enhanced Nv/C IV ratios is presented. It appears that some CVs have non-solar abundance material accreting onto the white dwarf suggesting an evolved secondary star while for others CO emission in the accretion disk may play a role. However, the exact mechanism or combination of factors causing the CO absorption anomaly in CVs is not yet clear.