  A considerable fraction of the central stars of planetary nebulae (CSPNe) are
hydrogen-deficient. As a rule, these CSPNe exhibit a chemical composition of
helium, carbon, and oxygen with the majority showing Wolf-Rayet-like emission
line spectra. These stars are classified as CSPNe of a spectral type [WC]. We
perform a spectral analysis of CSPN PB 8 with the Potsdam Wolf-Rayet (PoWR)
models for expanding atmospheres. The source PB 8 displays wind-broadened
emission lines from strong mass loss. Most strikingly, we find that its surface
composition is hydrogen-deficient, but not carbon-rich. With mass fractions of
55% helium, 40% hydrogen, 1.3% carbon, 2% nitrogen, and 1.3% oxygen, it differs
greatly from the 30-50% of carbon which are typically seen in [WC]-type central
stars. The atmospheric mixture in PB 8 has an analogy in the WN/WC transition
type among the massive Wolf-Rayet stars. Therefore we suggest to introduce a
new spectral type [WN/WC] for CSPNe, with PB 8 as its first member. The central
star of PB 8 has a relatively low temperature of T=52kK, as expected for
central stars in their early evolutionary stages. Its surrounding nebula is
less than 3000 years old, i.e. relatively young. Existing calculations for the
post-AGB evolution can produce hydrogen-deficient stars of the [WC] type, but
do not predict the composition found in PB 8. We discuss various scenarios that
might explain the origin of this unique object.
