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The shaping of the nebula is currently one of the outstanding unsolvedproblems in planetary nebula (PN) research. Several mechanisms have beenproposed, most of which require a binary companion. However, direct evidencefor a binary companion is lacking in most PNs. We have addressed this problemby obtaining precise radial velocities of seven bright proto-planetary nebulae(PPNs), objects in transition from the asymptotic giant branch to the PN phasesof stellar evolution. These have F-G spectral types and have the advantage overPNs of having more and sharper spectral lines, leading to better precision. Ourobservations were made in two observing intervals, 1991-1995 and 2007-2010, andwe have included in our analysis some additional published and unpublisheddata. Only one of the PPNs, IRAS 22272+5435, shows a long-term variation thatmight tentatively be attributed to a binary companion, with P $>$ 22 years, andfrom this, limiting binary parameters are calculated. Selection effects arealso discussed. These results set significant restrictions on the range ofpossible physical and orbital properties of any binary companions: they haveperiods greater than 25 years or masses of brown dwarfs or super-Jupiters.While not ruling out the binary hypothesis, it seems fair to say that theseresults do not support it.

Are Proto-Planetary Nebulae Shaped by a Binary? Results of a Long-Term Radial Velocity Study