The Masses and Evolutionary State of the Stars in the Dwarf Nova SS Cygni

The dwarf nova SS Cygni is a close binary star consisting of a K star transferring mass to a white dwarf by way of an accretion disk. We have obtained new spectroscopic observations of SS Cyg. Fits of synthetic spectra for Roche lobe-filling stars to the absorption-line spectrum of the K star yield the amplitude of the K star's radial velocity curve and the mass ratio, K K = 162.5 ± 1.0 km s -1 and q = M K / M WD = 0.685 ± 0.015. The fits also show that the accretion disk and white dwarf contribute a fraction f = 0.535 ± 0.075 of the total flux at 5500 Å. Taking the weighted average of our results with previously published results obtained using similar techniques, we find ⟨ K K ⟩ = 163.7 ± 0.7 km s -1 and ⟨ q ⟩ = 0.683 ± 0.012. The orbital light curve of SS Cyg shows an ellipsoidal variation diluted by light from the disk and white dwarf. From an analysis of the ellipsoidal variations, we limit the orbital inclination to the range 45° ≤ i ≤ 56°. The derived masses of the K star and white dwarf are M K = 0.55 ± 0.13 M ☉ and M WD = 0.81 ± 0.19 M ☉ , where the uncertainties are dominated by systematic errors in the orbital inclination. The K star in SS Cyg is 10%-50% larger than an unevolved star with the same mass and thus does not follow the mass-radius relation for zero-age main-sequence stars, nor does it follow the ZAMS mass-spectral type relation. Its mass and spectral type are, however, consistent with models in which the core hydrogen has been significantly depleted.