The lowest mass white dwarf

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 +/- 72 K and log g = 5.48 +/- 0: 03; with an estimated mass of 0.17 M circle dot , 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 circle dot. 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.