The Lowest Mass White Dwarf

Extremely low mass white dwarfs are very rare objects likely formed incompact binary systems. We present MMT optical spectroscopy of 42 low masswhite dwarf candidates serendipitously discovered in a survey for hypervelocityB-type stars. One of these objects, SDSS J0917+46, has Teff= 11,288 \pm 72 Kand log g = 5.48 \pm 0.03; with an estimated mass of 0.17 M_sun, it is thelowest gravity/mass white dwarf currently known. However, 40 of the low masscandidates are normal DA white dwarfs with apparently inaccurate SDSS gmagnitudes. We revisit the identification of low mass white dwarf candidatespreviously found in the SDSS, and conclude that four objects have M < 0.2M_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 theunseen companions.Comment: ApJ, accepted versio