Different types of close binary.
A close binary is a binary star system in which the separation of the components is comparable to the diameter of the stars. There are three main types, distinguished by the extent to which each star fills its Roche lobe.
In a detached binary, neither star fills its Roche lobe, so that there is no significant mass transfer between the components. In a semidetached binary, one of the stars fills its Roche lobe, which results in this star losing material in a matter stream that either falls directly onto its companion, or, as is more usual, that enters an accretion disk. In a contact binary both components fill their Roche lobes or, more often, overflow them so that there is a common convective envelope.