* Astronomy

Title: IGR J19308+0530: Roche lobe overflow on to a compact object from a donor 1.8 times as massive
Authors: E. M. Ratti, T. F. J. van Grunsven, M. A. P. Torres, P. G. Jonker, J. C. A. Miller-Jones, J. W. T. Hessels, H. Van Winckel, M. van der Sluys, G. Nelemans

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 semiamplitude 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 degrees. The accretor in IGR J19308+0530 is most likely a white dwarf, although a neutron star cannot entirely be excluded.

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