  Context: The commonly accepted mass-luminosity relation of central stars of
planetary nebulae (CSPNs) might not be universally valid. While earlier optical
analyses could not derive masses and luminosities independently (instead taking
them from theoretical evolutionary models) hydrodynamically consistent
modelling of the stellar winds allows using fits to the UV spectra to
consistently determine also stellar radii, masses, and luminosities without
assuming a mass-luminosity relation. Recent application to a sample of CSPNs
raised questions regarding the validity of the theoretical mass-luminosity
relation of CSPNs.
  Aims: The results of the earlier UV analysis are reassessed by means of a
simultaneous comparison of observed optical and UV spectra with corresponding
synthetic spectra.
  Methods: Using published stellar parameters (a) from a consistent UV analysis
and (b) from fits to optical H and He lines, we calculate simultaneous optical
and UV spectra with our model atmosphere code, which has been improved by
implementing Stark broadening for H and He lines.
  Results: Spectra computed with the parameter sets from the UV analysis yield
good agreement to the observations, but spectra computed with the stellar
parameters from the published optical analysis and using corresponding
consistent wind parameters show large discrepancies to both the observed
optical and UV spectra. The published optical analyses give good fits to the
observed spectrum only because the wind parameters assumed in these analyses
are inconsistent with their stellar parameters. By enforcing consistency
between stellar and wind parameters, stellar parameters are obtained which
disagree with the core-mass-luminosity relation for the objects analyzed. This
disagreement is also evident from a completely different approach: an
investigation of the dynamical wind parameters.
