Letter to the Editor

The double degenerate system NLTT 11748^*

¹ Astronomický ústav, Akademie věd České republiky, Fričova 298, 251 65 Ondřejov, Czech Republic e-mail: [kawka;vennes]@sunstel.asu.cas.cz
² Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation
³ Department of Physics & Astronomy, Francis Marion University, Box 100547, Florence, SC 29501, USA e-mail: tvaccaro@fmarion.edu

Received: 15 April 2010
Accepted: 18 May 2010

Abstract

We show that the extremely low-mass white dwarf NLTT 11748 (0.17 ) is in a close binary with a fainter companion. We obtained a series of radial velocity measurements of the low-mass white dwarf using the Hα core and determined an orbital period of 5.64 h. The velocity semi-amplitude (K = 274.8 km s^-1) and orbital period imply that it is a degenerate star, and that the minimum mass for the companion is 0.75  (assuming a mass of 0.167  for the primary). Our analysis of Balmer line profiles shows that a 0.75  white dwarf companion does not contribute more than 2% or 5% of the flux (V-band) for helium- or hydrogen-rich surfaces, respectively. The kinematics of the system suggest that it belongs to the Galactic halo.