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Binarity has been hypothesised to play an important, if not ubiquitous, rolein the formation of planetary nebulae (PNe). Yet there remains a severe paucityof known binary central stars required to test the binary hypothesis and toplace strong constraints on the physics of the common-envelope (CE) phase ofbinary stellar evolution. Large photometric surveys offer an unrivalledopportunity to efficiently discover many binary central stars. We have combinedphotometry from the OGLE microlensing survey with the largest sample of PNetowards the Galactic Bulge to systematically search for new binaries. A totalof 21 periodic binaries were found thereby more than doubling the known sample.The orbital period distribution was found to be best described by CE populationsynthesis models when no correlation between primary and secondary masses isassumed for the initial mass ratio distribution. A comparison with post-CEwhite dwarf binaries indicates both distributions are representative of thetrue post-CE period distribution with most binaries exhibiting periods lessthan one day. An estimated close binary fraction of 12--21% is derived and isthe first robust and independent validation of the prevailing 10--15% fractionestimated by Bond (2000). This suggests that binarity is not a precondition forthe formation of PNe and that close binaries do not play a dominant role in theshaping of nebular morphologies. Systematic effects and biases of the surveyare discussed with implications for future photometric surveys.

Binary Planetary Nebulae Nuclei towards the Galactic Bulge. I. Sample Discovery, Period Distribution and Binary Fraction