We present phase-resolved spectroscopy and photometry of the optical counterpart to the X–ray binary IGRÂ J19308+0530 .
Ellipsoidal modulations in the lightcurve 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 \sim 10-20 % disc contribution to the optical continuum .
We measure an orbital period of 14.662 \pm 0.001 hours , a radial velocity semi-amplitude for the companion star of K _ { 2 } = 91.4 \pm 1.4 \mathrm { km s ^ { -1 } } and a rotational broadening of v \sin i = 108.9 \pm 0.6 \mathrm { km s ^ { -1 } } .
From K _ { 2 } and v \sin i , given that the donor star is filling its Roche lobe , we derive a mass ratio of q = M _ { 2 } / M _ { 1 } = 1.78 \pm 0.04 , which is typically considered to be too large for stable Roche-lobe overflow .
Our observations support an inclination of \sim 50 degrees .
The accretor in IGRÂ J19308+0530 is most likely a white dwarf , although a neutron star can not entirely be excluded .