SY Cnc, a case for unstable mass transfer?

Abstract

Intermediate-resolution (0.5-1 Å) optical spectroscopy of the cataclysmic variable (CV) SY Cnc reveals the spectrum of the donor star. Our data enable us to resolve the orbital motion of the donor and provide a new orbital solution, binary mass ratio and spectral classification. We find that the donor star has spectral-type G8 ± 2 V and orbits the white dwarf with P = 0.382 3753 ± 0.000 0003 d, K2 = 88.0 ± 2.9 km s-1 and V sin i = 75.5 ± 6.5 km s-1. Our values are significantly different from previous works and lead to q = M 2/M1 = 1.18 ± 0.14. This is one of the highest mass ratios known in a CV and is very robust, because it is based on resolving the rotational broadening over a large number of metallic absorption lines. The donor could be a slightly evolved main sequence or descendant from a massive star which underwent an episode of thermal time-scale mass transfer. © 2009 RAS.