Ellipsoidal variation in EG Andromedae

We propose that the cause of the observed sinusoid in light curves of EG Andromedae is ellipsoidal variation due to tidal distortion, demonstrate that the amplitude and wave form agree well with observations, and analyse light curves to estimate fractional lobe filling by the red giant component. We find 82 to 100 per cent lobe filling, depending on assumed values of inclination, mass ratio and rotation. If our basic assumption is correct, the orbit must be far less eccentric than has been advocated in some recent publications, and is likely to be circular. The case of non-zero eccentricity fails by a wide margin to pass the Lucy-Sweeney test at the 5 per cent level of significance. We also solve radial velocity data simultaneously with the light curves under the circular orbit assumption. Either no rotation or synchronous rotation of the red giant allows good solutions. However, sub-synchronous rotation is a particularly interesting case because it may lead to mass transfer without lobe filling and also explain EG And's erratic photometric and spectroscopic behaviour. If our proposed mechanism of variation is correct, the red giant must be more than twice as large and more than four times as luminous as previously estimated, and the binary correspondingly more distant.