A close binary star in which both components fill their Roche lobes. When both components have just reached the bounding equipotential surface they are sometimes described as forming a double contact binary. More often, both components exceed the equipotential surface and share a common convective envelope. This configuration is sometimes described as an overcontact binary. Further expansion would carry the envelope to the equipotential surface that contains the outer (L₂) Lagrangian point, through which material would then be lost from the system.