  Post-AGB stars are the final stage of evolution of low-intermediate mass
stars (M<8 Solar Masses). Those in binary systems have stable circumbinary
discs. Using data from Herschel (PACS/SPIRE), we extend the SEDs of 50 galactic
post-AGB binary systems to sub-millimetre wavelengths and use the slope of the
SED as a diagnostic tool to probe the presence of large grains. Using a Monte
Carlo radiative transfer code (MCMax), we create a large grid of models to
quantify the observed spectral indices and use the presence of large grains in
the disc as a proxy for evolution.
