  We present high-speed ULTRACAM and SOFI photometry and X-shooter spectroscopy
of the recently discovered 94 minute orbital period eclipsing white dwarf /
main-sequence binary SDSS J085746.18+034255.3 (CSS 03170) and use these
observations to measure the system parameters. We detect a shallow secondary
eclipse and hence are able to determine an orbital inclination of 85.5 +/- 0.2
deg. The white dwarf has a mass of 0.51 +/- 0.05 Msun and a radius of 0.0247
+/- 0.0008 Rsun. With a temperature of 35,300 +/- 400K the white dwarf is
highly over-inflated if it has a carbon-oxygen core, however if it has a helium
core then its mass and radius are consistent with evolutionary models.
Therefore, the white dwarf in SDSS J085746.18+034255.3 is most likely a helium
core white dwarf with a mass close to the upper limit expected from evolution.
The main-sequence star is an M8 dwarf with a mass of 0.09 +/- 0.01 Msun and a
radius of 0.110 +/- 0.004 Rsun placing it close to the hydrogen burning limit.
The system emerged from a common envelope ~20 million years ago and will reach
a semi-detached configuration in ~400 million years, becoming a cataclysmic
variable with a period of 66 minutes, below the period minimum.
