2013MNRAS.431L..10R

We present phase-resolved spectroscopy and photometry of the optical counterpart to the X-ray binary IGR J19308+0530. Ellipsoidal modulations in the light curve show that the F-type companion star in the system is Roche lobe filling. The optical spectra are dominated by absorption features from the donor star, with ∼ 10-20 per cent disc contribution to the optical continuum. We measure an orbital period of 14.662±0.001 h, a radial velocity semi-amplitude for the companion star of K2 = 91.4±1.4 kms- 1 and a rotational broadening of vsini = 108.9±0.6 kms- 1. From K2 and vsini, given that the donor star is filling its Roche lobe, we derive a mass ratio of q = M2/M1 = 1.78±0.04, which is typically considered to be too large for stable Roche lobe overflow. Our observations support an inclination of ∼ 50°. The accretor in IGR J19308+0530 is most likely a white dwarf, although a neutron star cannot entirely be excluded.