Binarity of central stars of planetary nebulae

I list the 16 planetary nebulae (PNe) known to contain close-binary nuclei, and show that the nebulae generally have axisymmetric structures, including elliptical, bipolar, or ring morphologies. The orbital periods range from 2.7 hr to 16 days, and close binaries constitute approximate to 10% of all central stars. Since the known binaries were found mainly from photometric variability, which depends on heating effects at very small stellar separations, radial-velocity surveys will be necessary to find the large predicted population of binary nuclei with periods of about 10-100 days. Other PN phenomena that may arise from binary-star interactions include jets and point-symmetry, the periodically spaced arcs revealed by HST in the faint halos around several PNe and proto-PNe, and the existence of PNe in globular clusters. There is thus considerable circumstantial evidence that binary-star processes play a major role in the formation and shaping of many or even most PNe.