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[Abridged] [...] AIMS: We aim to establish the stellar parameters andmass-loss rates of the Galactic WC stars. These data provide the empiricalbasis of studies of (i) the role of WC stars in the evolution of massive stars,(ii) the wind-driving mechanisms, and (iii) the feedback of WC stars as inputto models of the chemical and dynamical evolution of galaxies. Methods: Weanalyze the nearly complete sample of un-obscured Galactic WC stars, usingoptical spectra as well as ultraviolet spectra when available. The observationsare fitted with theoretical spectra, using the Potsdam Wolf-Rayet (PoWR) modelatmosphere code. A large grid of line-blanked models has been established forthe range of WC subtypes WC4 - WC8, and smaller grids for the WC9 parameterdomain. Both WO stars and WN/WC transit types are also analyzed using specialmodels. Results: Stellar and atmospheric parameters are derived for more than50 GalacticWC and two WO stars, covering almost the whole GalacticWC populationas far as the stars are single, and un-obscured in the visual. In theHertzsprung-Russell diagram, theWC stars reside between the hydrogen and thehelium zero-age main sequences, having luminosities L from 10^4.9 to 10^5.6Lsun. The mass-loss rates scale very tightly with L^0.8. The two WO stars inour sample turn out to be outstandingly hot (\approx200 kK) and do not fit intothe WC scheme. Conclusions: By comparing the empirical WC positions in theHertzsprung-Russell diagram with evolutionary models, and from recent supernovastatistics, we conclude that WC stars have evolved from initial masses between20 solar masses and 45 Msun. In contrast to previous assumptions, it seems thatWC stars in general do not descend from the most massive stars. Only the WOstars might stem from progenitors that have been initially more massive than 45Msun.

The Galactic WC stars: Stellar parameters from spectral analyses indicate a new evolutionary sequence