Algol type (EA) binaries
Algol variables are a subgroup of eclipsing binary systems for which the moments of the beginning and end of the eclipses can be easily determined from the light curve. In other words, the ellipticity effect is either absent or insignificant because both stars (or at least the more luminous one) have nearly spherical shapes. The two eclipses can be comparable in depth or unequal, depending on the relative temperatures of the system's components. The brightness outside the eclipses may remain constant or may vary due to e.g. the reflection effect, ellipsoidal shape of the secondary component, third-body interaction, chromospheric activity, stellar pulsations, etc. Algol-type binaries can occur in either detached or semi-detached configuration. Such systems may consist of stars at various evolutionary stages, such as main-sequence stars, subgiants, giants, subdwarfs, or white dwarfs.
Binaries with eccentric orbits
In binary systems with eccentric orbits, the secondary eclipse is usually displaced from phase 0.5, and the two eclipses may be of unequal widths due to the changing orbital velocities of stars, as described by Kepler's second law.
Apsidal motion
The apsidal motion in eccentric binaries may "blur" one of the eclipses in the folded light curve, while the other eclipse remains precisely phased. This effect occurs when observations are collected over a long period of time. Below, we present four such light curves obtained within more than a decade of photometric monitoring conducted by the OGLE-IV survey.
Reflection effect
In close binary systems with significant temperature differences between the components, we can observe the so-called reflection effect. This phenomenon occurs when light from the hotter star is absorbed and re-emitted by the surface of the cooler star. The reflection effect induces brightness modulation outside the eclipses, reaching its maximum around the secondary eclipse.
In the OGLE database, we discovered extreme cases of eclipsing systems exhibiting a reflection effect. In the two light curves shown below, the amplitude of the reflection modulation exceeds 1 magnitude, the primary eclipses have a depth of several magnitudes, while the secondary eclipses are so shallow that they are almost invisible.
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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