The WD perspective on the PN binary hypothesis

There is no solid theoretical precept through which we can understand the amazing shapes of planetary nebulae (PN). The only plausible theoretical explanation of most PN shapes is that a companion interacts with the mass-losing progenitor of the central star of PN. Plausibility, however, is not sufficient to prove reality and efforts are ongoing to test the PN binary hypothesis observationally. Here, a short review is presented of the problem, but this time set within the context of binarity in the white dwarf (WD) population. We find that observationally the PN and WD binary populations agree. Most noticeably, both populations point to a deep flaw with current theories of the common envelope binary interaction. However, unexplained issues remain, several of which point to a sizable population of as yet undetected intermediate period binaries. Unfortunately, today the WD binaries do not constrain sufficiently the PN binary hypothesis.