When the orbital inclination i is sufficiently clone do 90″, we can nee the two components of a binary system eclipse each other. The graph of the apparent magnitude of the binary system against time is known as the LIGHT CURVE. In general the light curve of an eclipsing binary star displays two dips, or minima (the eclipses), in each orbital period – the deeper dip is called PRIMARY MINIMUM and the shallower one the SECONDARY MINIMUM. What information can we extract about the two stars from a study of the 1ight curve alone ? Often this is all the information we have available, although Home times it is possible to spectroscopic variability as well. of course, just as for a spectroscopic binary, the longitude of the node is in¬determinable, because we can rotate the picture of the eclipsing system on the sky about an axis along our line of sight and the same light curve. In addition we cannot find the direction of the orbital motion from the light curve alone. Moreover the shape of the light curve is unchanged if we keep the binary period constant but change the geometry of the whole system (the sizes of the stars and the binary separation) by, for example, a factor two. Consequently we cannot use the light curve, to determine the semi major axis of the orbit, but we can got Home, idea of the relative sizes of the two stars. In general the, light curves are difficult to interpret and so we shall start with a few simple examples before mentioning some of the difficulties and complication;, that can arise, together with the means of tackling them

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