The abundances of heavy elements in planetary nebulae reveal the evolutionary past of the central star, and the future of the interstellar medium, but suffer from a major systematic uncertainty: abundances derived from emission lines formed by recombination exceed those from collisionally excited lines by a factor ranging from ~2 to nearly three orders of magnitude in the most extreme case. The discrepancy has been known since the 1940s but only now are its causes being tracked down. There is now substantial evidence for the presence of cold hydrogen-deficient material within the normal hydrogen-rich gas of planetary nebulae, which gives rise to strongly enhanced recombination line emission.
It has recently become clear that the most extreme abundance discrepancies occur in nebulae which have a close binary central star. I will discuss my recent work which has doubled the sample of nebulae with a close binary central star and known chemistry, placing new constraints on the origin of hydrogen deficient material and the cause of the abundance discrepancy.