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Aim: We aim to study in detail the peculiar mineralogy and structure of thecircumstellar environment of two binary post-AGB stars, EPLyr and HD52961. Bothstars were selected from a larger sample of evolved disc sources observed withSpitzer and show unique solid-state and gas features in their infrared spectra.Moreover, they show a very small infrared excess in comparison with the othersample stars. Methods: The different dust and gas species are identified on thebasis of high-resolution Spitzer-IRS spectra. We fit the full spectrum toconstrain grain sizes and temperature distributions in the discs. This,combined with our broad-band spectral energy distribution and interferometricmeasurements, allows us to study the physical structure of the disc, using aself-consistent 2D radiative-transfer disc model. Results: We find that bothstars have strong emission features due to CO_2 gas, dominated by^{12}C^{16}O_2, but with clear ^{13}C^{16}O_2 and even ^{16}O^{12}C^{18}Oisotopic signatures. Crystalline silicates are apparent in both sources butproved very hard to model. EP Lyr also shows evidence of mixed chemistry, withemission features of the rare class-C PAHs. Whether these PAHs reside in theoxygen-rich disc or in a carbon-rich outflow is still unclear. With thestrongly processed silicates, the mixed chemistry and the low ^{12}C/^{13}Cratio, EP Lyr resembles some silicate J-type stars, although the depletedphotosphere makes nucleosynthetic signatures difficult to probe. We find thatthe disc environment of both sources is, to a first approximation, wellmodelled with a passive disc, but additional physics such as grain settling,radial dust distributions, and an outflow component must be included to explainthe details of the observed spectral energy distributions in both stars.

Analysis of the infrared spectra of the peculiar post-AGB stars EPLyr and HD52961