2013MNRAS.428.3543R

We present phase-resolved spectroscopy and photometry of a source discovered with the Chandra Galactic Bulge Survey (GBS), CXOGBS J174444.7-260330 (aka CX93 and CX153 in the previously published GBS list). We find two possible values for the orbital period P, differing from each other by ∼ 13 s. The most likely solution is P = 5.690 14(6) h. The optical lightcurves show ellipsoidal modulations, whose modelling provides an inclination of 32±1° for the most likely P. The spectra are dominated by a K5 V companion star (the disc veiling is ≲ 5 per cent). Broad and structured emission from the Balmer lines is also detected, as well as fainter emission from He i. From the absorption lines we measure K2 = 117±8kms- 1 and vsini = 69±7kms- 1. By solving the system mass function we find M1 = 0.8±0.2 M☉ for the favoured P and i, consistent with a white dwarf accretor, and M2 = 0.6±0.2 M☉. We estimate a distance in the range 400-700pc. Although in a low accretion state, both spectroscopy and photometry provide evidence of variability on a time-scale of months or faster. Besides finding a new, long orbital period cataclysmic variable (CV) in a low accretion state, this work shows that the design of the GBS works efficiently to find accreting X-ray binaries in quiescence, highlighting that the spectra of CVs in a low accretion state can at times appear suggestive of a quiescent neutron star or a black hole system.