  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 = 0.38 +- 0.06 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.
