Figure 19-21: Close Binary Star Systems The gravitational domain of a star in a close binary system is called its Roche lobe. The two Roche lobes meet at the inner Lagrangian point. The sizes of the stars relative to their Roche lobes determine whether the system is (a) a detached binary, (b) a semidetached binary, (c) a contact binary, or (d) an overcontact binary.