Close binary systems (CBS) are a separate class of binaries whose components are so close to each other that (at least during some stages of evolution) there is a direct physical interaction between them (exchange of mass and energy). They are divided into three subclasses:
a) detached,
b) semi-detached and
c) contact systems,
depending on whether none, one or both components are filling their respective Roche lobes. The figure to the right shows what close binaries look like. Due to tidal effects, the stars in close binaries can be significantly distorted or may significantly deviate from spherical symmetry that we are accustomed to.


Contact close binaries are systems in which both components are filling their inner Roche lobes. The stars that are initially close enough can come into contact by orbit shrinking due to loss of angular momentum or mass transfer, or due to various other mechanisms. Although there are contact CBS whose components are stars of early spectral types O, B and A, with radiative envelopes, the vast majority of contact systems are stars of W Ursae Majoris-type (W UMa) - systems with stars of late spectral types from F through K, which have a common convective envelope. One of the main features of these systems are approximately equal effective temperatures of stars, although the mass of the primary component is usually considerably larger than the mass of the secondary - from two (on average) up to ten times in the case of systems with extreme mass ratio q = M₂/M₁ << 1.

Extreme mass ratio close binaries of W UMa-type are interesting binary stars in which a "normal" one solar mass or slightly more massive star is in contact with a significantly less massive "dwarf" star. Earlier theoretical studies have shown that there is a minimum mass ratio q_min above which these CBS are stable and can be observed. If the mass ratio is lower than q_min, or equivalently if the orbital angular moment is only about three times the rotational angular momentum of the primary (more massive) component, the system becomes unstable (Darwin instability) and the components merge together forming a rapidly rotating single star (possibly a "blue straggler"). The observations, however, show that there are systems with mass ratio lower than q_min. According to the theory, such systems should not be observed.