Extremely low mass white dwarfs are very rare objects likely formed in compact binary systems .
We present MMT optical spectroscopy of 42 low mass white dwarf candidates serendipitously discovered in a survey for hypervelocity B-type stars .
One of these objects , SDSS J0917+46 , has T _ { eff } = 11,288 \pm 72 K and \log g = 5.48 \pm 0.03 ; with an estimated mass of 0.17 M _ { \sun } , it is the lowest gravity/mass white dwarf currently known .
However , 40 of the low mass candidates are normal DA white dwarfs with apparently inaccurate SDSS g magnitudes .
We revisit the identification of low mass white dwarf candidates previously found in the SDSS , and conclude that four objects have M < 0.2 M _ { \sun } .
None of these white dwarfs show excess emission from a binary companion , and radial velocity searches will be necessary to constrain the nature of the unseen companions .