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