RS Canum Venaticorum type binaries
RS Canum Venaticorum (RS CVn) stars are binary systems with at least one component exhibiting a high level of chromospheric activity, which manifest itself by the presence of large magnetic spots on the stellar surface. As a result, the eclipsing light curve is modulated by the so-called starspot wave generated by the rotation of the spotted star. The shapes and amplitudes of the starspot wave typically change over time as the spot coverage on the stellar surface evolves. Roughly 80% of RS CVn variables have their rotation period tidally synchronized with the binary orbital period, which implies that the period of the starspot wave is usually nearly equal to the interval between primary eclipses in the light curve. The small, usually a few percent, differences between wave and orbital periods can be attributed to the star's differential rotation.
Below, we present light curves of eight RS CVn stars from the OGLE Collection of Variable Stars. In each figure, the upper panel shows the unfolded I-band light curve collected between 2010 and 2019, while the lower left and right panels display the same light curve folded with the orbital period and the starspot-wave period, respectively. Note the flares visible in some of the light curves. This feature also indicates that these stars exhibit enhanced chromospheric activity.
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