Eclipsing binaries
The majority of stars in the Universe are components of binary or multiple star systems. When the binary orbit is nearly edge-on to our line of sight, we have the opportunity to observe mutual eclipses of the system's components. Eclipsing binaries constitute a very numerous category of variable stars. In the OGLE Collection of Variable Stars, we have already gathered over half a million such objects (e.g. Pietrukowicz et al. 2013, Pawlak et al. 2016, Soszyński et al. 2016).
The light curves of eclipsing stars show a wide variety of shapes. Naturally, their common characteristic is the occurrence of eclipses, usually two per orbital period, though some variables show only one eclipse per cycle due to their eccentric orbit. The deeper of the two minima, called the primary eclipse, occurs when the hotter star is eclipsed by the cooler component. The secondary (shallower) eclipse occurs in the opposite configuration. Eclipses in binary stars may be partial, annular, or total. Partial eclipses usually produce pointed minima, while annual and total eclipses result in flat-bottomed or round minima. Eclipses can be deeper or shallower, wider or narrower, depending on the system's geometry and the physical properties of the components. The brightness outside the eclipses may either remain constant or fluctuate, with these variations being directly linked to binarity or not.
Eclipsing binary systems can be divided into subtypes using various criteria, including the physical and evolutionary characteristics of the system components (main-sequence stars, red giants, white dwarfs, etc.), the degree of Roche-lobe filling (detached, semi-detached, and contact binaries), and the morphology of light curves (Algol, β Lyrae, and W Ursae Majoris variables).
Below, we present the light curves of several unusual eclipsing variables. More typical variables are shown on the following pages.
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