Zdeněk Kopal

THE ELLIPTICITY AND REFLEXION EFFECTS IN ECLIPSING BINARY SYSTEMS

The changes of light displayed by eclipsing binary systems provide us with the possibility of obtaining a vast amount of information concerning their nature. In studying the light curves attention has usually been paid to the conspicuous changes arising from the eclipses. The well known Russell-Shapley method and a number of its subsequent modifications enable one to derive from the light curve, primarily within minima, the geometrical elements of an eclipsing system, namely the radii of the components in terms of the orbital radius and the inclination of the orbital plane to the line of sight. Equally valuable information can, however, be inferred from the additional light changes displayed between eclipses. Were the variability of an eclipsing binary due to eclipses alone, its luminosity between the two minima should remain constant. In reality, this is approximately true only for very distant systems. As soon as the components move closer, the actual conditions become more complicated, and as a result the maximum brightness ceases to be constant. Two main effects responsible for the variation are known under the terms “ellipticity” and “reflexion” which are selfexplanatory. The ellipticity effect, essential for the (3 Lyrae type of variability, was recognized long ago. The reflection effect, usually far less conspicuous in amount, was discovered and correctly explained by R. S. Dugan* in 19013. A simple analytical treatment of both phenomena has been given in the classical papers by Russell and Shapley.+ A long series of papers on ellipticity and reflexion in close binary systems has appeared within the past twenty five years. It would be impossible to review them here in full. As regards reflexion, an important advance was made in 1926 when A. S. Eddington: and shortly afterward E. A. Milnet re-investigated the theory and physical processes underlying reflexion. The study of the ellipticity effect received