  SDSS J1507+52 is an eclipsing cataclysmic variable consisting of a cool,
non-radially pulsating white dwarf and an unusually small sub-stellar
secondary. The system has a high space velocity and a very short orbital period
of about 67 minutes, well below the usual minimum period for CVs. To explain
the existence of this peculiar system, two theories have been proposed. One
suggests that SDSS J1507+52 was formed from a detached white-dwarf/brown-dwarf
binary. The other theory proposes that the system is a member of the Galactic
halo-population.
  Here, we present ultraviolet spectroscopy of SDSS J1507+52 obtained with the
Hubble Space Telescope with the aim of distinguishing between these two
theories. The UV flux of the system is dominated by emission from the accreting
white dwarf. Fits to model stellar atmospheres yield physical parameter
estimates of T(eff) = 14200 \pm 500 K, log(g)=8.2 \pm 0.3, vsin(i)=180 \pm 20
kms-1 and [Fe/H]=-1.2 \pm 0.2. These fits suggest a distance towards SDSS
J1507+52 of d = 250 \pm 50 pc. The quoted uncertainties include systematic
errors associated with the adopted fitting windows and interstellar reddening.
Assuming that there is no contribution to the UV flux from a hot, optically
thick boundary layer, we find a T(eff) much higher than previously estimated
from eclipse analysis. The strongly sub-solar metallicity we infer for SDSS
J1507+52 is consistent with that of halo stars at the same space velocity. We
therefore conclude that SDSS J1507+52 is a member of the Galactic halo.
