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 ^ { -1 } .
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 \pm 15 km s ^ { -1 } .
At the adopted distance of 18 pc , the star has a very large space motion of 313 km s ^ { -1 } 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 , 000 K < T _ { eff } < 5 , 500 K , 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 ( \sim 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 \approx 0.45 M _ { \odot } .