We present phase-resolved spectroscopy of the short period cataclysmic variable WZ Sge obtained with the Hubble Space Telescope .
We were able to resolve the orbital motion of a number of absorption lines that likely probe the environment near the accreting white dwarf .
The radial velocities derived from simultaneous fits to 13 absorption lines indicate an orbital velocity semi-amplitude of K _ { UV } = 47 \pm 3 km/s .
However , we find that the phase zero is offset from the white dwarf ephemeris by +0.1 .
Our offset and velocity amplitude are very similar to constraints derived from optical emission lines from the quiescent accretion disk , despite the fact that we are probing material much closer to the primary .
If we associate the UV amplitude with K _ { 1 } , our dynamical constraints together with the K _ { 2 } estimates from Steeghs et al .
( 2001 ) and the known binary inclination of i = 77 \pm 2 imply 0.88 < M _ { 1 } < 1.53 M _ { \odot } , 0.078 < M _ { 2 } < 0.13 M _ { \odot } and 0.075 < q = M _ { 2 } / M _ { 1 } < 0.101 .
If we interpret the mean velocity of the UV lines ( -16 \pm 4 km/s ) as being due to the gravitational red-shift caused in the high- g environment near the white dwarf , we find v _ { grav } = 56 \pm 5 km/s which provides an independent estimate on the mass of the primary of M _ { 1 } = 0.85 \pm 0.04 M _ { \odot } when coupled with a mass-radius relation .
Our primary mass estimates are in excellent agreement and are also self-consistent with spectrophotometric fits to the UV fluxes despite the observed phase offset .
It is at this point unclear what causes the observed phase-offset in the UV spectra and by how much it distorts the radial velocity signature from the underlying white dwarf .