  It is already known that about 10% of central stars of PNe are very
short-period binaries (hours to days), which are detected through photometric
variations. These must have been formed through common-envelope interactions in
initially wide binaries, accompanied by ejection of the envelope and its
subsequent photoionization as a PN. Radial-velocity observations by ourselves
and others are now suggesting that an even larger fraction of planetary nuclei
may be spectroscopic binaries, making the total binary fraction very large.
However, we have not as yet been able to rule out the possibility that the
apparent velocity changes are actually due to stellar-wind variations. Pending
follow-up spectroscopic observations with large telescopes, it presently
appears plausible that binary-star ejection is the major formation channel for
planetary nebulae.
