Two new candidate ultra-compact X-ray binaries

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.