We noted above that when the two binary stars are spherical, the light curve remains constant outside of eclipses. However, when two stars are close enough together, the force of gravity is so high that each component raises huge tides on the other, so that both are pulled out to form elongated egg shapes. When the distortion is slight, each star is the shape of a prolate ellipsoid (stretched out along one axis). As the stars orbit one another, the long axis of each egg-shape turns around to face the other and so the surface area presented by each star to a fixed observer varies through the orbit, f n addition, those parts of the star that are furthest from its centre (i.e. the parts at the ends of the egg) do not shine as brightly as the rest. Even when the stars do not eclipse each other, their apparent brightness depends on the direction from which they are viewed and so varies with the orbital period P. Non-eclipsing systems that just fail to be eclipsing but show periodic light variations because of their distortion are called ELLIPSOIDAL VARIABLES.

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