YZ Leonis Minoris

YZ Leonis Minoris, also known as SDSS J0926+3624, is a star system in the constellation Leo Minor. It is an AM Canum Venaticorum-type variable star, a type of binary systems with ultra-short periods (between 5 and 70 minutes). It is also an eclipsing binary. The apparent magnitude of the system is generally 19.3m, varying by about two magnitudes due to periodic eclipses and outbursts. The distance to YZ LMi is of 815 pc.

Characteristics
YZ Leonis Minoris is made up of a white dwarf star and a low-mass donor companion. The white dwarf accretes matter from the companion via a helium-rich accretion disk. The disk around the white dwarf has a size ranging from 18,000 to 90,800 km, which is about 45% of the orbital separation of the components. The temperature of the disk varies from 5,000 K (in the outer parts of the disk) to 23,000 K (in the inner parts of the disk).

YZ Leonis Minoris is a very compact system. The orbital period of the stars is just 28 minutes, making it the eclipsing binary system with the shortest orbital period. The components are separated at a distance of 0.29 solar radius, and the surface-to-surface distance is 167,000 km. It is both an AM Canum Venaticorum variable and an eclipsing variable (eclipsing binary). The white dwarf is partially eclipsed by its donor companion.

The system has a normal apparent magnitude of 19.33m, which is way lower than the limit for naked -eye vision (6.5m), making it not visible to the naked eye. A recent estimate from Gaia DR3 gives a distance of 815 pc for YZ Leonis Minoris, which is significantly larger than previous estimates (of 460–470 pc).

White dwarf
The primary component of the system is a white dwarf. It has a mass between 0.79 and and a radius of. The white dwarf's surface gravity is about 200,000 times stronger than Earth's gravity. Its effective temperature is estimated to be at least 17,000K, and Sengupta et al. (2011) found three temperatures between 18,000 and 25,000 K. Its luminosity is about 0.009–0.035 times the solar luminosity. The white dwarf is accreting mass from the companion at a rate of 10$19.33$ solar masses per year, based on evolutionary models.

Low-mass companion
The secondary component of the system is a low-mass companion. It has a mass estimated to be between 0.027 and (28.3 and ) and its radius is estimated at 0.043 solar radius. The mass of the companion makes it semi-degenerate, it would be fully degenerate if its had a mass close to. Its temperature is estimated to be at 3570 K, with an upper limit of 3700 K. The luminosity of the companion is equivalent to 0.00035 times the solar luminosity.

Variability
SDSS J0926+2634 is an AM Canum Venaticorum-type variable star, which is a type of cataclysmic variable system that are ultracompact and deficient in hydrogen, with orbital periods of just some minutes. It is also an eclipsing binary, a type of binary stars where the components eclipse each other, causing variation in the apparent brightness. The American Association of Variable Star Observers also mentions YZ Leonis Minoris as a SU Ursae Majoris-type star (dwarf nova). YZ Leonis Minoris was the first system discovered that is both an eclipsing binary and an AM Canum Venaticorum star. As of 2022, more than 8 such systems are known.

The system presents eclipses every 28 minutes, which decrease the system's apparent magnitude by 2 magnitudes and last about two minutes, in addition to presenting outbursts that make the system's apparent magnitude increase by two magnitudes. YZ Leonis Minoris' mean apparent magnitude is 19.33m, decreasing to 17.11–16.81m during the outbursts. These outbursts happen every 100–200 days and are likely generated by bursts of enhanced mass transfer from donor star to the white dwarf.

YZ Leonis Minoris is the variable-star designation of the system.

Discovery
YZ Leonis Minoris was discovered in 2005 by Anderson et al. in a search for spectroscopically unusual objects, after an examination of spectra of 280,000 SDSS objects. It was discovered together with three other objects: SDSS J0129+3842, SDSS J1411+4812, and SDSS J1552+3201.