  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.
