Accreting white dwarfs among the planetary nebulae most luminous in [O III] λ5007 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] λ5007 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 [O III]-luminous PNs. This might explain the puzzling constant cutoff (maximum) [O III] λ5007 luminosity of the planetary nebula luminosity function across different galaxy types.