Dusty discs around binary post-AGB stars and their relation to protoplanetary discs.

We propose to obtain IRS spectra in the SH and LH mode of a sample of carefully selected binary post-AGB stars. Through detailed observing campaigns, we have recently established that these binaries are surrounded by a Keplerian dusty disc. The ISO-SWS spectra of only the few brightest examples, have shown that concerning mass, kinematics, mineralogy and dust processing, the circumstellar environment around the evolved stars resemble very much the protoplanetary discs around young stellar objects. They are therefore a unique laboratory to study the gas physics and dust mineralogy of Keplerian discs on quite different evolutionary timescales and radiation fields than during the star formation process. The detailed structure, let alone the formation, stability and evolution of the circumbinary discs around evolved stars are not well understood. With the new sample of IRS spectra, and our detailed modeling with state-of-the art radiative transfer codes based on modern laboratory data, we aim at a better understanding of the rich chemistry and strong processing of the different gas and dust components. Our carefully selected sample covers a significant range in orbital periods and understanding the properties of such Keplerian discs around binaries, will provide also insight in the role of binarity in the shapes and shaping of (proto) Planetary Nebulae.