  Context. The circumstellar environment of evolved stars is generally rich in
molecular gas and dust. Typically, the entire environment is either oxygen-rich
or carbon-rich, depending on the evolution of the central star. Aims. In this
paper we discuss three evolved disc sources with evidence of atypical emission
lines in their infrared spectra. The stars were taken from a larger sample of
post-AGB binaries for which we have Spitzer infrared spectra, characterised by
the presence of a stable oxygen-rich circumbinary disc. Our previous studies
have shown that the infrared spectra of post-AGB disc sources are dominated by
silicate dust emission, often with an extremely high crystallinity fraction.
However, the three sources described here are selected because they show a
peculiar molecular chemistry. Methods. Using Spitzer infrared spectroscopy, we
study in detail the peculiar mineralogy of the three sample stars. Using the
observed emission features, we identify the different observed dust, molecular
and gas species. Results. The infrared spectra show emission features due to
various oxygen-rich dust components, as well as CO2 gas. All three sources show
the strong infrared bands generally ascribed to polycyclic aromatic
hydrocarbons. Furthermore, two sample sources show C60 fullerene bands.
Conclusions. Even though the majority of post-AGB disc sources are dominated by
silicate dust in their circumstellar environment, we do find evidence that, for
some sources at least, additional processing must occur to explain the presence
of large carbonaceous molecules. There is evidence that some of these sources
are still oxygen-rich, which makes the detection of these molecules even more
surprising.
