CR Boötis

CR Boötis is an interacting binary system in the northern constellation of Boötes, abbreviated CR Boo. It is one of the best-known AM Canum Venaticorum stars. The system varies widely in brightness, ranging in apparent visual magnitude from 13.6 down to 17.5. The distance to this system is approximately 1,150 light years from the Sun, based on parallax measurements.

The variable luminosity of this object was discovered in 1983 by M. A. Wood and associates, with a light curve that is very similar to that of AM Canum Venaticorum. It was found to have an ultraviolet excess by the Palomar-Green survey and assigned the identifier PG 1346+082 in 1986. The system varies in both its photometric and spectroscopic properties, with a photometric quasi-period of 4–5 days. The optical spectrum displays only lines of helium.

Rapid flickering suggests this is a close binary system undergoing mass transfer, while emission by neutral helium indicates there is a thick accretion disk orbiting a compact object. The lack of X-ray emission means this accreting object is a white dwarf, rather than something more massive. The lack of hydrogen in the spectrum demonstrates that the donor star is a helium white dwarf with a lower mass than the primary. The pair have a brief orbital period of 0.017029 days with a mass ratio of 0.101$13$. The orbital plane is inclined at an angle of about 30° to the line of sight from the Earth.

This system displays two different states. The first is a lower state of quiescence that displays regular superoutbursts similar to ER Ursae Majoris on a roughly 46 day cycle. The second state is brighter with frequent outbursts. The system becomes bluer during superoutbursts, with a higher temperature.