The shaping of the nebula is currently one of the outstanding unsolved problems in planetary nebula ( PN ) research .
Several mechanisms have been proposed , most of which require a binary companion .
However , direct evidence for a binary companion is lacking in most PNs .
We have addressed this problem by obtaining precise radial velocities of seven bright proto-planetary nebulae ( PPNs ) , objects in transition from the asymptotic giant branch to the PN phases of stellar evolution .
These have F-G spectral types and have the advantage over PNs of having more and sharper spectral lines , leading to better precision .
Our observations were made in two observing intervals , 1991-1995 and 2007-2010 , and we have included in our analysis some additional published and unpublished data .
Only one of the PPNs , IRAS 22272+5435 , shows a long-term variation that might tentatively be attributed to a binary companion , with P > 22 years , and from this , limiting binary parameters are calculated .
Selection effects are also discussed .
These results set significant restrictions on the range of possible physical and orbital properties of any binary companions : they have periods 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 these results do not support it .