A&A 446, L17-L20 (2006)
DOI: 10.1051/0004-6361:200500229

Letter

Two new candidate ultra-compact X-ray binaries

C. G. Bassa¹, P. G. Jonker^{2, 3, 1}, J. J. M. in 't Zand^{2, 1} and F. Verbunt<sup>1

1</sup>  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 V=19.45, B-V=0.22 and R-I=0.22 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 R=22.15, R-I=1.53. 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.

Key words: X-rays: binaries -- X-rays: individual: 1A 1246-588, 4U 1812-12

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