A non-main-sequence secondary in SY Cancri

Abstract

Simultaneous spectroscopic and photometric observations of the Z Cam type dwarf nova SY Cancri were used to obtain absolute flux calibrations. A comparison of the photometric calibration with a wide slit spectrophotometric calibration showed that either method is equally satisfactory. A radial velocity study of the secondary star, made using the far red NaI doublet, yielded a mass ratio q = 0.68; this is very different from the value of 1.13 quoted in the literature. Using the new lower mass ratio, and constraining the mass of the white dwarf to be within reasonable limits, then leads to a mass for the secondary star that is substantially less than would be expected for its orbital period if it satisfied a main-sequence mass-radius relationship. We find a spectral type of M0 that is consistent with that expected for a main-sequence star of the low mass we have found. However, in order to fill its Roche lobe, the secondary must be significantly larger than a main sequence star of that mass and spectral type. The secondary is definitely not a normal main-sequence star.