The Detection of 13CO and Other Apparent Abundance Anomalies in the Secondary Stars of Long-Period Cataclysmic Variables

We present moderate-resolution (R>1800) infrared K-band spectra of 12 long-period (P_orb>6 hr) cataclysmic variables (CVs). We detect absorption lines from the photospheres of the secondary stars in every system, even though two of them were undergoing outbursts. We have attempted to assign a spectral type to each of the secondary stars, and these classifications are generally consistent with previous determinations. We find evidence for abundance anomalies that include enhancements and/or deficits for all of the species commonly found in K-band spectra of G- and K-type dwarfs. There is, however, only one common abundance anomaly: extremely weak CO features. Only two of the 12 objects appeared to have normal levels of CO absorption. We interpret this as evidence of low carbon abundances. In addition, we detect ¹³CO absorption in four of the 12 objects. Depleted levels of ¹²C and enhanced levels of ¹³C indicate that material that has been processed in the CNO cycle is finding its way into the photospheres of CV secondary stars. In systems with luminous accretion disks, we find that the spectrum of the secondary star is contaminated by a source that flattens (reddens) the continuum. While free-free or classical accretion disk spectra are flatter than the blackbody-like spectra of G and K dwarfs, removal of such contamination from the K-band data results in spectra in which the absorption features become too strong to be consistent with those of G and K dwarfs.