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 12CO and 13CO bands at 2.3525 and 2.3448 microns, 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 13CO in both objects, we attribute it instead to a cluster of neutral atomic absorption features (primarily due to TiI) 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 13CO bands at 2.3739 and 2.4037 microns, which is insensitive to spectral contamination by star-spots. 
V.S. Dhillon, S.P. Littlefair, T.R. Marsh, M.J. Sarna, E.H. Boakes