Figure 11-30Detached, Semidetached, Contact, and Overcontact Binaries (a) In a detached binary, neither star fills its Roche lobe. (b) If one star fills its Roche lobe, the binary is semidetached. Mass transfer is often observed in semidetached binaries. (c) In a contact binary, both stars fill their Roche lobes. (d) The two stars in an overcontact binary both overflow their Roche lobes. The two stars actually share the same outer atmosphere.