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We present phase-resolved spectroscopy and photometry of the opticalcounterpart to the X-ray binary IGR J19308+0530. Ellipsoidal modulations in thelight curve show that the F-type companion star in the system is Roche lobefilling. The optical spectra are dominated by absorption features from thedonor star, with ~10-20 per cent disc contribution to the optical continuum. Wemeasure an orbital period of 14.662+-0.001 h, a radial velocity semiamplitudefor the companion star of K2 = 91.4+-1.4 kms-1 and a rotational broadening ofvsini = 108.9+-0.6 kms-1. From K2 and vsini, given that the donor star isfilling its Roche lobe, we derive a mass ratio of q = M2/M1=1.78+-0.04, whichis typically considered to be too large for stable Roche lobe overflow. Ourobservations support an inclination of ~50 degrees. The accretor in IGRJ19308+0530 is most likely a white dwarf, although a neutron star cannotentirely be excluded.

IGR J19308+0530: Roche lobe overflow on to a compact object from a donor 1.8 times as massive