  The common envelope interaction is responsible for evolved close binaries.
Among them are a minority of central stars of planetary nebula (PN). Recent
observational results, however, point to most PN actually being in binary
systems. We therefore ask the question if it is feasible that most, or even all
Galactic PN derive from a common envelope interaction. Our recent calculation
finds that if all single and binary primary stars with mass between ~1-8 Mo
eject a PN, there would be many more PN in the galaxy than observed. On the
other hand, the predicted number of post-common envelope PN is more in
agreement with the total number of PN in the Galaxy. This is a new indication
that binary interactions play a functional role in the creation of PN and an
encouragement to intensify efforts to detect binary companions.
