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 _ { \text { eff } } = 14200 \pm 500 K , \log g = 8.2 \pm 0.3 , v \sin i = 180 \pm 20 km s ^ { -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 _ { \text { eff } } much higher than previously estimated from eclipses 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 .