A new extremely low-mass white dwarf in the NLTT catalogue

We report on the discovery of the extremely low-mass, hydrogen-rich white dwarf, NLTT 11748. Based on measurements of the effective temperature (8540 +/- 50 K) and surface gravity (log g = 6.20 +/- 0.15) obtained by fitting the observed Balmer line profiles with synthetic spectra, we derive a mass of 0.167 +/- 0.005 M(circle dot). This object is one of only a handful of white dwarfs with masses below 0.2 M(circle dot) 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 Gyr for the white dwarf, we infer a progenitor mass of 0.87-0.93 M circle dot. The likely companion has yet to be identified, but a search for radial velocity variations may help constrain its nature.