Letter to the Editor

An ultra-cool white dwarf serendipitously found with COMBO-17

Department of Physics, Denys Wilkinson Bldg., University of Oxford, Keble Road, Oxford, OX1 3RH, UK e-mail: cwolf@astro.ox.ac.uk

Received: 14 October 2005
Accepted: 26 October 2005

Abstract

We report the discovery of an ultra-cool white dwarf in the COMBO-17 survey. So far, only seven objects have been discovered in this rare category of white dwarfs, which are characterized by strong flux depression in the far-red and near-infrared part of the spectrum, presumably due to collisionally induced absorption (CIA). The new object COMBO-17 J114356.08-0144032 has very similar colours to LHS 3250, which was the first of its kind to be recognized. However, at it is the faintest and possibly most distant such object discovered to date. It is the only such object in COMBO-17 at ; due to the small sky coverage of 0.78  this chance discovery can not provide any constraints on the abundance of faint ultra-cool white dwarfs. We speculate on the basis of the proper motion that this new object is probably a member of the Galactic (thin) disk.