Semidetached binary stars are the ones where one of the components fills its Roche lobe and the other does not. Gas from the surface of the Roche lobe filling component (donor) is transferred to the other star (accretor). The mass transfer dominates the evolution of the system. In many cases, the inflowing gas forms an accretion disk around the accretor. Examples: X-ray binaries and Cataclysmic variable stars.