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 \pm 1.0 \textrm { km s$ { } ^ { -1 } $ } and q = M _ { K } / M _ { wd } = 0.685 \pm 0.015 .
The fits also show that the accretion disk and white dwarf contribute a fraction f = 0.535 \pm 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 \langle \textrm { K } _ { K } \rangle = 163.7 \pm 0.7 \textrm { km s$ { } ^ { -1 } $ } and \langle q \rangle = 0.683 \pm 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 ^ { \circ } \leq i \leq 56 ^ { \circ } .
The derived masses of the K star and white dwarf are M _ { K } = 0.55 \pm 0.13 M _ { \odot } and M _ { wd } = 0.81 \pm 0.19 M _ { \odot } , where the uncertainties are dominated by systematic errors in the orbital inclination .
The K star in SS Cyg is 10 % to 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 .