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We present phase-resolved spectroscopy of the short period cataclysmicvariable WZ Sge obtained with the Hubble Space Telescope. We were able toresolve the orbital motion of a number of absorption lines that likely probethe environment near the accreting white dwarf. The radial velocities derivedfrom simultaneous fits to 13 absorption lines indicate an orbital velocitysemi-amplitude of K_UV = 47 +/- 3 km/s. However, we find that the phase zero isoffset from the white dwarf ephemeris by +0.1. Our offset and velocityamplitude are very similar to constraints derived from optical emission linesfrom the quiescent accretion disk, despite the fact that we are probingmaterial 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+/-2 imply 0.88<M_1<1.53 M_sun,0.078 < M_2 < 0.13 M_sun and 0.075<q=M_2/M_1<0.101. If we interpret the meanvelocity of the UV lines (-16+/-4 km/s) as being due to the gravitationalred-shift caused in the high-g environment near the white dwarf, we findv_grav=56+/-5 km/s which provides an independent estimate on the mass of theprimary of M_1=0.85+/-0.04 M_sun when coupled with a mass-radius relation. Ourprimary mass estimates are in excellent agreement and are also self-consistentwith spectrophotometric fits to the UV fluxes despite the observed phaseoffset. It is at this point unclear what causes the observed phase-offset inthe UV spectra and by how much it distorts the radial velocity signature fromthe underlying white dwarf.

Dynamical constraints on the component masses of the cataclysmic variable WZ Sge