Paper detail

SY Cancri, a case for unstable mass transfer?

Intermediate resolution (0.5-1 Angs) optical spectroscopy of the cataclysmic variable 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=0.3823753 +- 0.0000003 day, K2=88.0 +-2.9 km/s and V sin i=75.5 +- 6.5 km/s. Our values are significantly different from previous works and lead to q=M2/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-timescale mass transfer.