β Lyrae-type (EB) binaries
The apparent brightness of β Lyrae-type binary stars changes continuously, making it difficult to specify the exact moments of the beginning and end of the eclipses. Additionally, the primary and secondary eclipses differ significantly in depth, indicating different surface temperatures of both components. The brightness variations outside the eclipses are due to the strong deformation of one or both components of the system by the tidal forces from the companion, causing the stars to take on an ellipsoidal shape. One of the stars may even fill its Roche lobe, leading to mass transfer toward the companion. Such systems are called semidetached binaries.
O'Connell effect
Some eclipsing binaries exhibit a phenomenon known as the O'Connell effect, where the maxima of the light curve are not equally bright. While the exact cause of this effect remains unclear, it is most likely associated with the mass transfer between the components. In most cases, the brighter maximum follows the primary (deeper) eclipse. Such a phenomenon is called the positive O'Connell effect. The opposite scenario, known as the negative O'Connell effect, is much less common. The last of the four example light curves demonstrates the negative O'Connell effect.
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