2001A&A...378L..41H

We report the discovery of the ``21''µm and ``30''µm features in the planetary nebulae around the hydrogen-deficient stars HD 826 and HD 158269. The carriers of these features are known to be produced in outflows around carbon-rich stars. This discovery demonstrates that the bulk of the dust in these nebulae has been produced during a carbon-rich phase before the atmospheres of these stars became hydrogen poor. This is the first time that the ``21''µm feature has been detected in any planetary nebula. It shows that once formed its carrier can survive the formation of the nebula and the exposure to the UV radiation of the hot central star. This means that the carrier of ``21''µm feature is not transient: the absence of the feature sets limits on the production of its carrier.