  We derive the conditions for a backflow toward the central star(s) of
circumstellar material to occur during the post-asymptotic giant branch (AGB)
phase. The backflowing material may be accreted by the post-AGB star and/or its
companion, if such exists. Such a backflow may play a significant role in
shaping the descendant planetary nebula, by, among other things, slowing down
the post-AGB evolution, and by forming an accretion disk which may blow two
jets. We consider three forces acting on a slowly moving mass element: the
gravity of the central system, radiation pressure, and fast wind ram pressure.
We find that for a significant backflow to occur, a slow dense flow should
exsist, such that the relation between the total mass in the slow flow, M, and
the solid angle it covers, Omega, is given by (4*pi*M/Omega)>0.1Mo. The
requirement for both high mass loss rate per unit solid angle and a very slow
wind, such that it can be decelerated and flow back, probably requires close
binary interaction.
