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Context. The uncertainty in the degree to which radiation-driven winds of hotstars might be affected by small inhomogeneities in the density leads to acorresponding uncertainty in the determination of the atmospheric mass lossrates from the strength of optical recombination lines. Furthermore, theoptical recombination lines also react sensitively to even small changes in thedensity structure resulting from the (often assumed instead of computed)velocity law of the outflow. This raises the question of how reliable theparameter determinations from such lines are.  Results. The discrepancy between the optical and the UV analyses is confirmedto be the result of a missing consistency between stellar and wind parametersin the optical analysis. The influence of the density (velocity field) is ofthe same order as that of moderate clumping factors. Moderate clumping factorsleave the UV spectra mostly unaffected, indicating that the influence on theionization balance, and thus on the radiative acceleration, is small. Insteadof the erratic behavior of the clumping factors claimed from the opticalanalyses, our analysis based on the velocity field computed from radiativedriving yields similar clumping factors for all CSPNs. The shock temperaturesand the ratios of X-ray to bolometric luminosity required to reproduce thehighly ionized Ovi line agree with those known from massive O stars, againconfirming the similarity of O-type CSPN and massive O star atmospheres.  Conclusions. The similarity of the winds of O-type CSPNs and those of massiveO stars justifies using the same methods based on the dynamics ofradiation-driven winds in their analysis, thus supporting the earlier resultthat several of the CSPNs in the sample have near-Chandrasekhar-limit massesand may thus be possible single-star progenitors of type Ia supernovae.

Radiation-driven winds of hot luminous stars. XVIII. The reliability of  stellar and wind parameter determinations from spectral analysis of selected  central stars of planetary nebulae and the possibility of single-star SN Ia  progenitors