CRYSTALLINE SILICATES IN PLANETARY-NEBULAE WITH [WC] CENTRAL STARS

We present ISO-SWS spectroscopy of the cool dusty envelopes surroundin g two Planetary Nebulae with [WC] central stars, BD+30 3639 and He 2-1 13. The lambda < 15 mu m region is dominated by a rising continuum wit h prominent emission from C-rich dust (PAHs), while the long wavelengt h part shows narrow solid state features from crystalline silicates. T his demonstrates that the chemical composition of both stars changed v ery recently (less than 1000 years ago). The most likely explanation i s a thermal pulse at the very end of the AGE or shortly after the AGE. The H-rich nature of the C-rich dust suggests that the change to C-ri ch chemistry did not remove all H. The present-day H-poor [WC] nature of the central star may be due to extensive mass loss and mixing follo wing the late thermal pulse.