Accreting White Dwarfs among the Planetary Nebulae Most Luminous in [O   III]5007 Emission
  I propose that some of the most luminous planetary nebulae (PNs) are actually proto-PNs, where a companion white dwarf (WD) accretes mass at a relatively high rate from the post-asymptotic giant branch star that blew the nebula. The WD sustains a continuous nuclear burning and ionizes the nebula. The WD is luminous enough to make the dense nebula luminous in the [O III]5007 line, In young stellar populations these WD accreting systems account for a small fraction of [O III]-luminous PNs, but in old stellar populations these binaries might account for most, or even all, of the [OIII]-luminous PNs. This might explain the puzzling constant cutoff (maximum) [O III]5007 luminosity of the planetary nebula luminosity function across different galaxy types. 
Noam Soker (Technion, Israel)