Observations show that at least half of all stars are members of multiple systems , Let us, therefore, touch briefly on such systems (detailed reviews can be found in Lubow and Artymowicz  and Mathieu et al. ). The discs in binaries occur in two principal types: circumbinary (GG Tau, DQ Tau, UZ TauE, UY Aur, HD 98800, BD+31 643) and circumstellar (HR 4796A, T Tau N, GG Tau, UZ TauW) (see Jensen and Mathieu ; Mathieu et al.  and references therein). UZ Tau and GG Tau make intriguing examples of systems, in which both types of discs are found [161,162].
There are also planets found in binaries — 16 CygB, 55 Cnc, r Boo [7,163]. It is well known that three-body stellar systems are unstable and tend to eject the lowest mass object (e.g. [164-166]). Benest [167,168] found that stable planetary orbits exist up to distances from each star of about half the binary's periastron separation. Stable circumbinary orbits also exist, when the semimajor axis, measured in units of the binary's semimajor axis, is larger than 2 . Planets migrating to larger distances should be ejected from the system.
The gravitational field of close companion(s) perturbs also the orbits of dust particles. The orbits are unstable at circumstellar distances of the same order as the component separation in a multiple star system. Therefore discs in such systems are expected to be truncated (on the outside of circumstellar discs and on the inside of circumbinary discs) by tidal interactions [170,171]. A decrease in the disc mass is then expected in systems with a separation smaller than the size of a typical disc. Indeed, from millimetre observations discs in close binary systems (the separation is less than ~ 100 AU) were found to be substantially less massive than the discs around single stars or in wide systems (e.g. [17,172-175]). In some cases evidence also exists of a tidal truncation of a circumstellar disc [76,78,162,176] and of a dynamically cleared central region in a circumbinary disc [100,160,177,178]. At the same time, for the survival times of the discs, no significant difference between singles and binaries was found .
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