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.
Accretion disks
Mass transfer between the components of a binary system can produce interesting effects visible in the time-series photometry. In the light curves below, the wider eclipse occurs when one of the stars is obscured by the accretion disk surrounding the other star. Since the accretion disk is larger than the star, one of the eclipses lasts significantly longer than the second one.
Double Periodic Variables
Mass transfer in binary systems is linked to a phenomenon discovered by Mennickent et al. (2003), who analyzed OGLE light curves of blue stars in the Magellanic Clouds. These so-called Double Periodic Variables (DPV) are semi-detached binary systems that display two photometric cycles: the typical eclipsing (or ellipsoidal) modulation and a long-period cycle that lasts 20 to 50 times longer than the orbital period. While the origin of this long-period modulation remains unclear, it has been suggested that it may be related to a magnetic dynamo in the donor star. In the figures below, the left panels show the original light curves of DPV stars folded with the orbital period, while the middle and right panels display the disentangled light curves phased with the orbital and the long period, respectively.
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