  We report the discovery of PM J13420-3415, a faint (V=17) white dwarf with a
very high proper motion mu = 2.55 arcsec/yr. The star was found in the southern
sky extension of the SUPERBLINK proper motion survey. A red spectrum shows the
classical signature of a DA white dwarf, with a weak H-alpha line in absorption
as the only prominent feature. The star is also found to have a large radial
velocity V_rad = +212+/-15 km/s. At the adopted distance of 18 pc, the star has
a very large space motion of 313 km/s relative to the Sun. An integration of
the space motion shows that the star is on a nearly polar Galactic orbit, and
is thus an unambiguous member of the Galactic halo. However, with an estimated
effective temperature 5,000K < T_eff < 5,500K, the white dwarf appears to be
much younger than expected for a denizen of the halo. The apparent paradox can
be explained if the white dwarf is the relatively young (~2 Gyr) remnant of a
longer-lived (10-14 Gyr) main sequence star, in which case the object is
predicted to be a low-mass white dwarf with M ~ 0.45 M_sun.
