YY Canis Minoris

YY Canis Minoris, abbreviated YY CMi and otherwise referred to as HD 67110, is an eclipsing contact binary in the constellation of Canis Minor, close to the border with Hydra. Its apparent magnitude ranges between 8.46 and 9.23, making it too faint to be seen by the naked eye but visible using binoculars. It is located at a distance of approximately according to Gaia DR3 parallax measurements.

Stellar properties

The system consists of two F-type stars more massive and hotter than the Sun. Both objects are larger than typical F-type main-sequence stars, which may be because they are evolving away from the main sequence. As such, they both received the luminosity class V-IV, with their spectra indicating an intermediate luminosity between main-sequence stars and subgiants.
The two stars orbit each other every 1.094 days, separated by a mere 6.41 solar radii. As a result, both of them are gravitationally distorted and overfilling their Roche lobes, becoming a W Ursae Majoris variable.

YY CMi was first reported to be variable in August 1934 by Otto Morgenroth of the Sonneberg Observatory, who described it as an Algol variable with a magnitude range of 8.4 to 8.8. The 1958 edition of the General Catalogue of Variable Stars listed it as a Beta Lyrae variable with a combined spectral type of F5. Krishna Damodar Abhyankar gave the spectral types F6III and A5V to the two components, though this was noted to be incorrect in 1970 and, in 1981, the two were reclassified as evolved stars of roughly the types F6 and early G comprising a contact binary. This evolved and contact nature of the system has been supported by subsequent studies, but a re-examination of Abhyankar's light curves in 1999 yielded slightly earlier evolutionary stages and spectral types for the stars.