Letter to the Editor

Two new candidate ultra-compact X-ray binaries

¹ Astronomical Institute, Utrecht University, PO Box 80 000, 3508 TA Utrecht, The Netherlands e-mail: c.g.bassa@astro.uu.nl
² SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS83, Cambridge, Massachusetts, USA

Received: 2 December 2005
Accepted: 21 December 2005

Abstract

We present the identification of the optical counterparts to the low-mass X-ray binaries 1A 1246-588 and 4U 1812-12. We determine the X-ray position of 1A 1246-588 from ROSAT/PSPC observations and find within the error circle a blue star with , and which we identify as the counterpart. Within the Chandra error circle of 4U 1812-12, a single star is present which appears blue with respect to the stars in the vicinity. It has , . Distance estimates for both systems indicate that the optical counterparts are intrinsically faint, suggesting that they are ultra-compact X-ray binaries. These identifications would increase the number of candidate ultra-compact X-ray binaries from 2 to 4, whereas orbital periods are measured for only 7 systems in the Galactic disk.