  Context. The uncertainty in the degree to which radiation-driven winds of hot
stars might be affected by small inhomogeneities in the density leads to a
corresponding uncertainty in the determination of the atmospheric mass loss
rates from the strength of optical recombination lines. Furthermore, the
optical recombination lines also react sensitively to even small changes in the
density structure resulting from the (often assumed instead of computed)
velocity law of the outflow. This raises the question of how reliable the
parameter determinations from such lines are.
  Results. The discrepancy between the optical and the UV analyses is confirmed
to be the result of a missing consistency between stellar and wind parameters
in the optical analysis. The influence of the density (velocity field) is of
the same order as that of moderate clumping factors. Moderate clumping factors
leave the UV spectra mostly unaffected, indicating that the influence on the
ionization balance, and thus on the radiative acceleration, is small. Instead
of the erratic behavior of the clumping factors claimed from the optical
analyses, our analysis based on the velocity field computed from radiative
driving yields similar clumping factors for all CSPNs. The shock temperatures
and the ratios of X-ray to bolometric luminosity required to reproduce the
highly ionized Ovi line agree with those known from massive O stars, again
confirming the similarity of O-type CSPN and massive O star atmospheres.
  Conclusions. The similarity of the winds of O-type CSPNs and those of massive
O stars justifies using the same methods based on the dynamics of
radiation-driven winds in their analysis, thus supporting the earlier result
that several of the CSPNs in the sample have near-Chandrasekhar-limit masses
and may thus be possible single-star progenitors of type Ia supernovae.
