Planetary nebulae : getting closer to an unbiased binary fraction
  Why 80% of planetary nebulae are not spherical is not yet understood. The Binary Hypothesis states that a companion to the progenitor of the central star of a planetary nebula is required to shape the nebula and even for a planetary nebula to be formed at all. A way to test this hypothesis is to estimate the binary fraction of central stars of planetary nebula and to compare it with the main sequence population. Preliminary results from photometric variability and infrared excess techniques indicate that the binary fraction of central stars of planetary nebulae is higher than that of the putative main sequence progenitor population, implying that PNe could be preferentially formed via a binary channel. This article briefly reviews these results and future studies aiming to refine the binary fraction. 
D. Douchin, O. De Marco, G. H. Jacoby, T. C. Hillwig, D. J. Frew, I.   Bojicic, G. Jasniewicz and Q. A. Parker
astro-ph.SR