  We report the detection of a radial velocity companion to the extremely low
mass white dwarf LP400-22. The radial velocity of the white dwarf shows
variations with a semi-amplitude of 119 km/s and a 0.98776 day period, which
implies a companion mass of M > 0.37 Msun. The optical photometry rules out a
main sequence companion. Thus the invisible companion is another white dwarf or
a neutron star. Using proper motion measurements and the radial velocity of the
binary system, we find that it has an unusual Galactic orbit. LP400-22 is
moving away from the Galactic center with a velocity of 396 km/s, which is very
difficult to explain by supernova runaway ejection mechanisms. Dynamical
interactions with a massive black hole like that in the Galactic center can in
principle explain its peculiar velocity, if the progenitor was a triple star
system comprised of a close binary and a distant tertiary companion. Until
better proper motions become available, we consider LP400-22 to be most likely
a halo star with a very unusual orbit.
