This paper reviews our knowledge on binary central stars of planetary nebulae
and presents some personal opinions regarding their evolution. Three types of
interactions are distinguished: type I, where the binary companion induces the
mass loss; type II, where it shapes the mass loss but does not enhance it; type
III, where a wide orbit causes the centre of mass to move, leading to a spiral
embedded in the wind. Surveys for binary central stars are discussed, and the
separations are compared to the distribution for binary post-AGB stars. The
effect of close binary evolution on nebular morphology is discussed.
Post-common-envelope binaries are surrounded by thin, expanding disks, expelled
in the orbital plane. Wider binaries give rise to much thicker expanding torii.
Type I binary evolution predicts a wide distribution of masses of central
stars, skewed towards low masses. Comparison with observed mass distributions
suggests that this is unlikely to be the only channel leading to the formation
of a planetary nebula. A new sample of compact Bulge nebulae shows about 40% of
nebulae with binary-induced morphologies.Comment: Invited review, in 'Evolution and chemistry of symbiotic stars and
related objects', Wierzba, August 2006. To appear in Baltic Astronom
