Global magnetic fields are effective in shaping the AGB wind into axi-symmetric geometries (e.g., Garcia-Segura et al. 2005 ), and have therefore been thought to be the main agent shaping non spherical PN. However, it appears that single AGB stars cannot access enough energy to power a global magnetic field during the entire super-wind phase of the envelope ejection ( Nordhause et al. 2007). Magntic fields take about 100 years to drain the differential stellar rotation that is needed to sustain them. This is too short a time for the magnetic field to shape the AGB mass-loss. An alternative is that a companion, not too far from the AGB star, shapes the super-wind either directly by garavity or by resupplying the angular momentum necessary to sustain a global magnetic field that can in turn shape the outflow.

What we know about binary central stars

Surveys such as these are biased against wider binaries because as orbital separations increase, irradiation effects, ellipsoidal variability or the chance of eclipses all dimish. Bond (2000) assumed this to be the reason why all the binaries found by this survey have periods smaller than ~3 days; he therefore concluded that the central star of PN close binary fraction of 10-15% is a lower limit. De Marco et al. 2008 argued theoretically that the bias of such survey shoould be about 2 weeks and that if systems with 3 day < P < 2 weeks had been present they should have been found by Bond (2000). Miszalski et al. (2009) confirmed this prediction observationally, concluding that the common envelope interaction, responsible for the creation of all close evolved stars, inclusing central stars of PN, results by and large in binaries with period smaller than a couple of days.

This discovery imposes novel and stringent constraints for the PN binarity hypothesis. With a close binary fraction of 12-21% and the knowledge of the period distribution, 79-88% of all non spherical PNe have to be explained by a mechanism other than a commmon evnelope interaction where the binary survived (although we should keep in mind that some post-common envelope binary central stars might not have been detected by the variability survey because of effects that reduce the irradiated vs. non-irradiated light contrast such as phase locking). Some PN could have been shaped by a common envelope interaction that resulted in a merger. Others still, from an interaction with a wider binary companion that avoided the common envelope. With regard to the latter class of binary interactions, we recall that 30% of post-AGB stars are single lined spectroscopic binaries with 100 days < P < 1500 days (where the upper limit is dictated by the radial velocity technique bias; van Winckel 2003), and assuming that all the post-AGB observed will make a PN, then ~30% of all central stars should be binaries with periods in that range. This would mean that the short + intermediate period binary fraction is at least 42-61%.

A second area where the Miszalski et al (2009) binary fraction and period distribution has application is the physics common envelope interaction itself. Almost no population synthesis model (e.g., Yungelson et al. 1993; Han et al. 1995 ) predicts such short post-common envelope periods (the only one that does - de Kool 1992 - is one of the oldest models and makes some questionable assumptions).

How to detect sub-luminous, distant or non-existent companions (mergers)?

Near-IR excess surveys are possibly the best technique to find companions to central stars, since they can detect companions at arbitrary distances. A caveat is that central stars are bright at all wavelengths, making faint cool compansions difficult to detect. Frew and Parker 2008 determined a central star near-IR excess fraction ascribable to a companion to be 53%. However, this number does not include late M companions while it might include objects too wide to have influenced PN shaping. A more sensitive survey needs to be carried out.

Indirect detection of past merger events need to be also developed (e.g., Tout et al. 2008).