YZ Cassiopeiae (21 Cas) is a star system away from Earth, in the constellation Cassiopeia. It comprises three stars: an eclipsing Algol-type binary and a visually fainter star about 3000 AU distant. It is faintly visible to the naked eye under good observing conditions.
The primary star in the YZ Cassiopeiae system is a white subgiant (main sequence) star of spectral type A1Vm and 2.31 solar masses () with a less massive main sequence dwarf star of type F2VJerzy M. Kreiner, Chun-Hwey Kim, Il-Seong Nha. YZ CAS: Atlas of O-C Diagrams of Eclipsing Binary Stars and 1.35 .
In 1924, Joel Stebbins announced his discovery that the star, then known as 21 Cassiopeiae, is an eclipsing binary. It was given its variable star designation, YZ Cassiopeiae, in 1925. The apparent magnitude of the eclipsing binary varies from 5.65 to 6.05 with a period of 4.4672 days. Combined, they appear to have a spectral type of A2IV.
The binary has a dimmer (magnitude 9.7 according to Norton, or 10.5 by SIMBAD) companion of 0.8 orbiting with a period of about 86 580 years.
References Tokovinin 1997-1999. J/A+AS/124/75. Multiple star catalogue (MSC)