2 PLANETARY-NEBULA CANDIDATES AROUND HOT DA WHITE-DWARFS

Two possible planetary nebulae have been found during a search around hot H-rich white dwarfs. These are located close to Ton 320 and RE 173 8 + 665. Both are extremely faint, but are rather brighter in [N II] t han Halpha. This would bring the total number of nebulae found around independently identified hot DA white dwarfs to four, with one other a round a PG 1159 star. In addition, we failed to detect any around six PG 1159 stars. We suggest that this lack is very difficult to explain if all white dwarfs undergo essentially the same evolution, each passi ng through a PG 1159 stage. Furthermore, the most widely quoted altern ative, in which approximately 25% undergo a late He-shell flash causin g a brief return to the AGB, can successfully explain this only if it produces a substantially greater delay than is currently assumed.