Post-Asymptotic Giant Branch (post-AGB) stars are objects that are in the
short transition  between the AGB and the Planetary Nebula phase.
Observations of these objects  provide important information on the
knowledge of this phase of evolution, and help to constrain and improve
theoretical evolutionary models.

The known sample of post-AGB objects however is strongly biased towards
objects that left the AGB only a few hundred years ago, far less than the
expected transition time of  1000 to even 10,000 years.  This difference is
basically due to a selection effect; searches in the IRAS Point Source
Catalog always required at least a detection at 12 micron, biasing
towards objects with hot dust, close to the star.
The aim of this proposal is to study a very carefully selected sample of
candidate post-AGB stars with very cool dust shells, which we believe to
represent the older,  not previously studied, post-AGB stars.  To study
their photospheres we have obtained extensive sets of optical and
ultraviolet spectroscopy.
Now we wish to study their circumstellar environment using ISO,  which
is the only platform with the sensitivity and wavelength coverage to do
this.  We propose to obtain medium resolution SWS-01 spectra combined
with PHOT-P and PHOT-C photometry.  The observations will be used to derive
the properties of the circumstellar environment of the objects, in particular
the mass loss rates, the chemistry of the circumstellar envelopes, and
the kinematic ages of the dust shells. These objects are expected to be
much further in their (post-AGB) evolution than the existing sample of
post-AGB stars.  The observations will be compared to post-AGB objects
listed in the Central Program, which will yield important information
on the effects of the evolution of the circumstellar shells during the
post-AGB phase.  In addition, the observations can be used to constrain
models that predict the transition time scales from the AGB to the PN phase.