  We report the discovery of a new white dwarf/M dwarf binary, HS1857+5144,
identified in the Hamburg Quasar Survey (HQS). Time-resolved optical
spectroscopy and photometry were carried out to determine the properties of
this new cataclysmic variable progenitor pre-CV). The light curves of
HS1857+5144 display a sinusoidal variation with a period of Porb=383.52 min and
peak-to-peak amplitudes of 0.7 mag and 1.1 mag in the B-band and R-band,
respectively. The large amplitude of the brightness variation results from a
reflection effect on the heated inner hemisphere of the companion star,
suggesting a very high temperature of the white dwarf. Our radial velocity
study confirms the photometric period as the orbital period of the system. A
model atmosphere fit to the spectrum of the white dwarf obtained at minimum
light provides limits to its mass and temperature of Mwd=~0.6-1.0 Msun and
Twd=~70000-100000 K, respectively. The detection of HeII 4686 absorption
classifies the primary star of HS1857+5144 as a DAO white dwarf. Combining the
results from our spectroscopy and photometry, we estimate the mass of the
companion star and the binary inclination to be Msec=~0.15-0.30 Msun and
i=~45-55 deg, respectively. We classify HS1857+5144 as one of the youngest
pre-CV known to date. The cooling age of the white dwarf suggests that the
present system has just emerged from a common envelope phase ~10^5 yr ago.
HS1857+5144 will start mass transfer within or below the 2-3h period gap.
