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We present K-band echelle spectra of the cataclysmic variable SS Cyg and thepre-cataclysmic variable V471 Tau in order to measure the strengths of the 12COand 13CO bands at 2.3525 and 2.3448 microns, respectively, and so perform theobservational test of the common-envelope model of close binary star evolutionproposed by Sarna et al. (1995). Although we find evidence of an absorptionfeature coincident with the expected wavelength of 13CO in both objects, weattribute it instead to a cluster of neutral atomic absorption features(primarily due to TiI) possibly arising from star-spots on the surfaces of therapidly rotating secondary stars in these systems, thereby rendering the testinconclusive. We present a modified observational test of common-envelopeevolution, based on the observation of the 13CO bands at 2.3739 and 2.4037microns, which is insensitive to spectral contamination by star-spots.Comment: 6 pages, 2 figures, accepted for publication in A&

On the use of $^\mathsf{12}$CO/$^\mathsf{13}$CO as a test of common-envelope evolution