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

¹ Department of Physics and Astronomy, University of Sheffield, Sheffield, S3 7RH, UK e-mail: vik.dhillon@sheffield.ac.uk
² Department of Physics and Astronomy, University of Southampton, Highfield, Southampton, SO17 1BJ, UK e-mail: trm@astro.soton.ac.uk
³ N. Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland e-mail: sarna@camk.edu.pl

Corresponding author: V. S. Dhillon, vik.dhillon@shef.ac.uk

Received: 7 September 2001
Accepted: 16 July 2002

Abstract

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 μm, respectively, and so perform the observational test of the common-envelope model of close binary star evolution proposed by Sarna et al. ([CITE]). 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 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 ¹³CO bands at 2.3739 and 2.4037 μm, which is insensitive to spectral contamination by star-spots.