Observed Planetary Nebulae as Descendants of Interacting Binary Systems
  We examine recent studies on the formation rate of planetary nebulae and find this rate to be about one-third of the formation rate of white dwarfs. This implies than only about one-third of all planetary nebulae that evolve to form white dwarfs are actually bright enough to be observed. This finding corresponds with the claim that it is necessary for a binary companion to interact with the asymptotic giant branch stellar progenitor for the descendant planetary nebulae to be bright enough to be detected. The finding about the formation rate also strengthens De Marco's conjecture that the majority of observed planetary nebulae harbor binary systems. In other words, single stars almost never form observed planetary nebulae. 
Noam Soker (Technion, Israel)