A contact binary is a highly evolved 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, as shown in the accompanying diagram.
This configuration is sometimes as an overcontact
binary. Further expansion would carry the envelope to the equipotential
surface that contains the outer (L2) Lagrangian
point, through which material woudl then be lost from the system.