  We investigate the development of bipolar outflows during the early post-AGB
evolution. A sample of ten OH/IR stars is observed at high angular resolution,
including bipolar nebulae (OH231.8+4.2), bright post-AGB stars (HD 101584) and
reflection nebulae (e.g. Roberts 22). The IRAS colour--colour diagram separates
the sample into different types of objects. One group may contain the
progenitors to the (few) extreme bipolar planetary nebulae. Two objects show
colours and chemistry very similar to the planetary nebulae with late IR-[WC]
stars. One object is a confirmed close binary.
  A model is presented consisting of an outer AGB wind which is swept up by a
faster post-AGB wind, with either wind being non-spherically symetric. The
interface of the two winds is shown to exhibit a linear relation between
velocity and distance from the star. The OH data confirms the predicted linear
velocity gradients, and reveals torus-like, uniformly expanding components. All
sources are discussed in detail using optical/HST images where available. ISO
data for Roberts 22 reveal a chemical dichotomy, with both crystalline
silicates and PAHs features being present. IRAS 16342-3814 shows a dense torus;
HST data shows four point-like sources located symmetrically around the nebula,
near the outer edge of the dense torus.
  Lifetimes for the bipolar OH/IR stars are shown to be in excess of 10^4 yr,
longer than normal post-AGB timescales. This suggests that the disks are
near-stationary. We suggest that accretion from such a disk slows down the
post-AGB evolution. Such a process could explain the link between the
long-lived bipolar nebular geometry and the retarded star.
