  FUSE spectroscopy has proved that extremely hot hydrogen-deficient post-AGB
stars (PG1159 stars) display matter on their surface that usually remains
hidden in the region between the H- and He-burning shells of the former AGB
star. Hence, the spectral analysis of PG1159 stars allows to study directly the
chemistry of this intershell region which is the outcome of complicated burning
and mixing processes during AGB evolution. Detailed abundance determinations
provide constraints for these processes which are still poorly understood. With
FUSE we have discovered high neon and fluorine overabundances. There is also a
significant iron deficiency, which may be caused by s-process neutron capture
transforming iron into heavier elements.
