  We present ISO-SWS spectroscopy of the cool dusty envelopes surrounding two
Planetary Nebulae with [WC] central stars, BD+30~3639 and He~2-113. The lambda
< 15micron region is dominated by a rising continuum with prominent emission
from C-rich dust (PAHs), while the long wavelength part shows narrow solid
state features from crystalline silicates. This demonstrates that the chemical
composition of both stars changed very recently (less than 1000 years ago). The
most likely explanation is a thermal pulse at the very end of the AGB or
shortly after the AGB. The H-rich nature of the C-rich dust suggests that the
change to C-rich 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
following the late thermal pulse.
