Testing common-envelope binary evolution with measurements of the 12C/13C isotopic ratios

We carried out medium resolution spectroscopy of close, detached white dwarf - red dwarf binaries without planetary nebulae and cataclysmic variables in the infra-red K-band (2.3 - 2.4 mum). We have detected the (CO)-C-12 bands at 2.3535 mum and 2.3829 mum and, for the first time, the (CO)-C-13 bands at 2.3448 mum and 2.3739 mum from the secondary stars. Comparison with synthetic spectral profiles of these lines indicate that the C-12/C-13 isotopic ratio in these binaries are different from solar. Our preliminary estimates of the isotopic ratios agree well with the theoretical predictions from the common-envelope binary evolution model by Marks et al. (1997). Thus we may have the first direct proof that these objects have indeed undergone common-envelope evolution.