AE Phoenicis, a W Ursae Majoris contact binary 165 ly from Sol
A co-orbiting pair of stars whose inner atmospheres or surfaces touch, often called a W Ursae Majoris binary, after the system of the same name 162 ly from Sol. In most contact binaries one component is larger than the other. In this kind of system both stars fill their respective Roche Lobes; many contact binaries in fact exceed the equipotential boundary defined by the Roche Lobes, and become so-called overcontact binaries with a common gaseous envelope.
A contact binary pair can have a system of planets in orbit around it, but the innermost planet must orbit no closer than 3.5 times the separation of the two stars.
Most W Ursae Majoris binaries have strong magnetic fields due to the disrupted nature of the component stars; this can make any planetary system associated with such stars an extreme environment.