On the use of 12CO/13CO as a test of common-envelope evolution

We present K-band echelle spectra of the cataclysmic variable SS Cyg and the pre-cataclysmic variable V471 Tau in order to measure the strengths of the (CO)-C-12 and (CO)-C-13 bands at 2.3525 and 2.3448 mum, respectively, and so perform the observational test of the common-envelope model of close binary star evolution proposed by Sarna et al. (1995). Although we find evidence of an absorption feature coincident with the expected wavelength of (CO)-C-13 in both objects, we attribute it instead to a cluster of neutral atomic absorption features (primarily due to Ti I) possibly arising from star-spots on the surfaces of the rapidly rotating secondary stars in these systems, thereby rendering the test inconclusive. We present a modified observational test of common-envelope evolution, based on the observation of the (CO)-C-13 bands at 2.3739 and 2.4037 mum, which is insensitive to spectral contamination by star-spots.