Spectroscopy and Component Masses of the Eclipsing Dwarf Nova HS 0907 + 1902

HS0907+1902 was recently discovered to be one of a handful of deeply eclipsing dwarf novae with periods longward of the 2 – 3 hr ‘gap’. This paper presents orbit-resolved spectra and time series photometry of an eclipse. The apparent velocity amplitude of the M-dwarf secondary is K2 = 297 ± 15 km s . The phase of the radial velocities of the Hα emission line wings agrees accurately (for once) with the phase of the whitedwarf motion deduced from the eclipse, and an estimate of the emission-line velocity amplitude yields K1 = 115 ± 7 km s . The eclipse width is ∆φ = 0.060 ± 0.005. At face value, these measurements yield mass estimates of M1 = 0.99 ± 0.12 M⊙ for the white dwarf and M2 = 0.38 ± 0.06 M⊙ for the secondary. The eclipse width and nominal mass ratio constrain the binary inclination i to be 77.3± 0.9. The influence of systematic uncertainties on these values is discussed; the conclusion that the white dwarf is somewhat more massive than typical field white dwarfs appears to be robust. The Hα emission line profile out of eclipse is only slightly double-peaked, but the line shows a strong rotational disturbance in eclipse. Models of the line profile through eclipse using a flat, Keplerian disk do not give a good quantitative match to the observations. Subject headings: cataclysmic variables — stars: fundamental parameters — stars: individual (HS0907+1902) — binaries: eclipsing — binaries: spectroscopic — white dwarfs