We report the detection of a radial velocity companion to SDSS J091709.55+463821.8 , the lowest mass white dwarf currently known with M \sim 0.17 M _ { \odot } .
The radial velocity of the white dwarf shows variations with a semi-amplitude of 148.8 \pm 6.9 km s ^ { -1 } and a period of 7.5936 \pm 0.0024 hours , which implies a companion mass of M \geq 0.28 M _ { \odot } .
The lack of evidence of a companion in the optical photometry forces any main-sequence companion to be smaller than 0.1 M _ { \odot } , hence a low mass main sequence star companion is ruled out for this system .
The companion is most likely another white dwarf , and we present tentative evidence for an evolutionary scenario which could have produced it .
However , a neutron star companion can not be ruled out and follow-up radio observations are required to search for a pulsar companion .