We report on the discovery of the extremely low-mass , hydrogen-rich white dwarf , NLTT 11748 .
Based on measurements of the effective temperature ( 8540 \pm 50 K ) and surface gravity ( \log { g } = 6.20 \pm 0.15 ) obtained by fitting the observed Balmer line profiles with synthetic spectra , we derive a mass of 0.167 \pm 0.005 M _ { \odot } .
This object is one of only a handful of white dwarfs with masses below 0.2 M _ { \odot } that are believed to be the product of close binary evolution with an episode of Roche lobe overflow onto a degenerate companion ( neutron star or white dwarf ) .
Assuming membership in the halo population , as suggested by the kinematics and adopting a cooling age of 4.0 - 6.3 Gyrs for the white dwarf , we infer a progenitor mass of 0.87 - 0.93 M _ { \odot } .
The likely companion has yet to be identified , but a search for radial velocity variations may help constrain its nature .