Dynamical Constraints on the Component Masses of the Cataclysmic Variable WZ Sagittae

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+/-3) km s^-1. 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 K₁, our dynamical constraints together with the published K₂ estimates and the known binary inclination of i=77+/-2 imply 0.88 M_solar<M₁<1.53 M_solar, 0.078 M_solar<M₂<0.13 M_solar, and 0.075<q=M₂/M₁<0.101. If we interpret the mean velocity of the UV lines [-(16+/-4) km s^-1] as being due to the gravitational redshift caused in the high-g environment near the white dwarf, we find v_grav=(56+/-5) km s^-1, which provides an independent estimate on the mass of the primary of M₁=(0.85+/-0.04) M_solar 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.