There appears to be several similarities between two classes
of chemically-peculiar stars: the Fe-deficient post-AGB stars
and the barium and Tc-poor S stars. They are binaries with non-zero
eccentricities and periods ranging from one to several years,
with one component having evolved off the AGB. Current scenarios for
the evolution of low-mass binary systems indicate that barium and
S systems may be the descendants of the post-AGB binaries. Moreover,
dust seems to play a key role in these systems.
As far as Fe-poor post-AGB stars are concerned,
their photospheric abundance peculiarities resemble the elemental
depletions in the gas phase of the ISM. It became thus clear that
their low Fe contents are not primordial. They were instead acquired
in the course of the binary evolution by a process of re-accretion of
circumstellar gas, depleted in refractory elements which
remained trapped in the non-accreted dust phase.
As far as barium and S stars are concerned, the two closest systems
exhibit complex lightcurves which share some similarities with
those of binary post-AGB stars. In particular, the observed light
variations suggest that dust may be scattering the red-giant light
at some orbital phases and obscuring it at other phases.
The eclipsing-like behaviour has been attributed to dust as well, since
the present white dwarf companion is too cool and too faint to contribute
significantly to the system light. Existing IR data already suggest
that a moderate amount of dust is present in these barium and S systems.
By deriving the energy distribution of the dust with unprecedented
detail and accuracy using ISOPHOT and SWS, this program
aims at a better understanding of the properties of the dust (size,
chemical type, location in the system and mass loss history) present
in these two classes of chemically-peculiar binary stars. It should thus
also shed some light on the exact relationships between these classes.