  While most of the low-mass stars stay hydrogen-rich on their surface
throughout their evolution, a considerable fraction of white dwarfs as well as
central stars of planetary nebulae have a hydrogen-deficient surface
composition. The majority of these H-deficient central stars exhibit spectra
very similar to massive Wolf-Rayet stars of the carbon sequence, i.e. with
broad emission lines of carbon, helium, and oxygen. In analogy to the massive
Wolf-Rayet stars, they are classified as [WC] stars. Their formation, which is
relatively well understood, is thought to be the result of a (very) late
thermal pulse of the helium burning shell.
  It is therefore surprising that some H-deficient central stars which have
been found recently, e.g. IC 4663 and Abell 48, exhibit spectra that resemble
those of the massive Wolf-Rayet stars of the nitrogen sequence, i.e. with
strong emission lines of nitrogen instead of carbon. This new type of central
stars is therefore labelled [WN]. We present spectral analyses of these objects
and discuss the status of further candidates as well as the evolutionary status
and origin of the [WN] stars.
