Spectroscopy and Component Masses of the Eclipsing Dwarf Nova HS0907+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 K_2 = 297 +- 15 km/s. The phase of the radial velocities of the H-alpha emission line wings agrees accurately (for once) with the phase of the white-dwarf motion deduced from the eclipse, and an estimate of the emission-line velocity amplitude yields K1 = 115 +- 7 km/s. The eclipse width is delta-phi = 0.060 +- 0.005. At face value, these measurements yield mass estimates of M1 = 0.99 +- 0.12 solar masses for the white dwarf and M2 = 0.53 +- 0.08 solar masses for the secondary. The eclipse width and nominal mass ratio constrain the binary inclination to be 77.3 +- 0.9 degree. 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 H-alpha 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