Constraining photoshperic abundances of donor stars in cataclysmic variables

Advances in near-infrared (NIR) spectroscopy have recently allowed the direct detection of absorption lines in the atmospheres of the mass donor stars in many Cataclysmic Variable (CV) systems, in which a white dwarf accretes a main sequence star via Roche lobe overflow. Many curious systems have appeared across many of the CV subtypes, showing extremely weak or completely absent CO absorption bands in the NIR K-band but Na I and/or Ca I features appropriate for their expected spectral types. It is a challenge to fit such a donor star into the standard evolutionary paradigm of CVs, though recent works have shown that the theoretical considerations governing many aspects of CV and binary evolution are too simplistic. We have undertaken a large and systematic search across the pandimensional stellar parameter space of Teff, log 10(g), [Fe/H], [C/H], and 12C/13C ratio. Synthetic K-band spectra are computed using a customized list of atomic and molecular transitions, model atmospheres from the MARCS group, and a modified version of the popular spectral synthesis program MOOG. The models are compared to the observations using a combination of both grid search and genetic algorithm techniques to assess the range of parameter variation possible. We have also examined a number of standard reference stars to verify the functionality of our techniques and parameter estimates. We will show that GK Per, SS Cyg, RU Peg, CH UMa, MU Cen, and EM Cyg all exhibit a substantially subsolar C abundance, with SS Cyg additionally showing a 12C/13C ratio indicative of nuclear evolution of the mass donating star. We will present our current results in this ongoing program to uncover the abundances in donor stars in CVs, allowing us to learn more about their evolutionary history.