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, demons trate that the amplitude and wave form agree well with observations, a nd analyse light curves to estimate fractional lobe filling by the red giant component. We find 82 to 100 per cent lobe filling, depending o n 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 circul ar. The case of non-zero eccentricity fails by a wide margin to pass t he 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 rotat ion of the red giant allows good solutions. However, sub-synchronous r otation is a particularly interesting case because it may lead to mass transfer without lobe filling and also explain EG And's erratic photo metric and spectroscopic behaviour. If our proposed mechanism of varia tion is correct, the red giant must be more than twice as large and mo re than four times as luminous as previously estimated, and the binary correspondingly more distant.