Time-resolved spectroscopy and photometry of the dwarf nova FS Aurigae in quiescence

We present results of non-simultaneous time-resolved photometric and spectroscopic observations of the little-studied dwarf nova FS Aur in quiescence. The spectrum of FS Aur shows strong and broad emission lines of hydrogen and He I, and of weaker He II lambda4686 and CIII/N III blend, similar to other quiescent dwarf novae. All emission lines are single-peaked, however their form varies with orbital phase. Absorption lines from a late-type secondary are not detected. From the radial velocity measurements of the hydrogen lines Hbeta and Hgamma we determined a most probable orbital period P = 0.(d)059 +/- 0.(d)002. This period agrees well with the 0.(d)0595 +/- 0.(d)0001 estimate by Thorstensen et al. (1996). On the other hand, the period of photometric modulations is longer than the spectroscopic period and can be estimated as 3 hours. Longer time coverage during a single night is needed to resolve this problem. Using the semi-amplitude of the radial velocities, obtained from measurements of hydrogen and helium lines, and some empirical and theoretical relations we limited the basic parameters of the system: a mass ratio q greater than or equal to 0.22, a primary mass M-1 = 0.34-0.46 M-., a secondary mass M-2 less than or equal to 0.1 M-., and an inclination angle i = 51degrees-65degrees. Doppler tomography has shown at least two bright regions in the accretion disk of FS Aur. The first, brighter spot is located at phase about 0.6. The second spot is located opposite the first one and occupies an extensive area at phases about 0.85-1.15.