In an X-ray binary system, in which one of the two objects is black hole candidate, there are several ways to exchange mass. A paper states that" their host systems are mass-exchange binaries containing a nondegenerate star that supplies gas to the black hole via a stellar wind or via Roche-lobe overflow in a stream that emanates from the inner Lagrangian point. The mass-donor star in the Roche-lobe overflow systems is typically a low mass (~one sun mass) sun-like star, and the X-ray source is transient, alternating between yearlong outbursts and years or decades of quiescence. the wind-fed X-ray source, on the other hand, are fueled by massive hot stars( larger than 10 times of sun mass) and are persistently luminous." I wonder why the Roche-lobe overflow has a period while the other mechanism has not.