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 ^ { -1 } and a 0.98776 day period , which implies a companion mass of M \geq 0.37 M _ { \odot } .
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 \pm 43 km s ^ { -1 } , 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 .