Before the ISO mission was launched the commonly accepted theory of dust
formation in post-AGB objects was that carbon-rich nebulae formed carbon dust
(either PAH's, amorphous carbon or graphite) and oxygen-rich nebulae formed a
mixture of minerals commonly called `silicates'. Also oxygen-rich chemistry
should not take place in carbon-rich nebulae.
Once the first SWS and LWS spectra from ISO became available it soon became
clear that this theory was too simple. The first indication was the LWS
spectrum of NGC 7027 (a carbon-rich nebula) were a clear detection of both an
OH and an H20 line was found. Also in the spectra of the oxygen-rich nebulae
NGC 6302 and Roberts 22 both PAH features and crystalline oxygen-rich dust
features were discovered.
The fact that in the early part of the ISO mission already three cases have
been discovered which clearly refute the simple theory of dust formation in AGB
ejecta, is suggestive of the fact that this phenomenon might be quite common.
Therefore we propose, as a follow-up to the guaranteed time programme of
Beintema et al., a more systematic search for planetary nebulae with similar
characteristics as described above. Only ISO can provide the wavelength
coverage needed for this research.