V523 Cassiopeiae (V523 Cas) is an eclipsing binary star system located in the constellation of Cassiopeia. It is classified as an overcontact binary of the W Ursae Majoris (W UMa) type. It has one of the shortest known orbital periods among non-degenerate contact binary systems.
Observation V523 Cas was first identified as a variable star by F. Weber in 1957 through photographic observations. It was subsequently classified as an EW-type eclipsing binary by H. Haussler in 1974 based on its light curve morphology, which shows two unequal minima indicative of a contact configuration where the secondary eclipse is shallower than the primary. Photometric monitoring began in the 1980s, with extensive B and V-band observations conducted at Lowell Observatory over three consecutive nights in 1983, revealing the system's short period and stability during that epoch. Radial-velocity spectroscopy, initiated in August 1981, confirmed the binary nature and refined the spectral type to approximately K5. Subsequent campaigns in the 1990s and 2000s utilized multi-filter CCD photometry (B, V, R, I) and near-infrared (JHKs) observations to model the light curves and derive system parameters. The system has been part of long-term period studies spanning nearly a century, with times of minimum light compiled from visual, photographic, and photoelectric data. It is detectable as an X-ray source, attributed to coronal activity from the chromospheres of its active K-dwarf components.
Structure The contact binary consists of a thermally disequilibrated pair of K0V and M2V main-sequence stars, overall spectral type K4V, that share a common envelope and are in physical contact, resulting in mutual eclipses that produce its observed variability. The secondary also features strong starspot activity. There is a companion star, also of type M2V, that orbits the main star on a roughly 115-year orbit. There may be a fourth star, another red dwarf, with an orbital period of around 19 years. The 5.61 h period of the binary is one of the shortest known among contact binaries, but is lengthening by 7.64 ms every year; the tertiary also induces a sinusoidal variation with amplitude 54±7 min in sync with its orbit. Mass is transferring from the secondary to the primary of the contact binary at a rate of about 9.17×10−8 M☉/y, faster than the system can thermalize, and it should continue for a few million years until the pair separates and the secondary is much smaller than the primary.
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