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We aim to constrain the properties and evolutionary status of early andmid-spectral type supergiants (from O4 to O7.5). These posses the highestmass-loss rates among the O stars, and exhibit conspicuous wind profiles. Usingthe non-LTE wind code CMFGEN, we simultaneously analyzed the FUV-UV and opticalspectral range to determine the photospheric properties and wind parameters. Wederived effective temperatures, luminosities, surface gravities, surfaceabundances, mass-loss rates, wind terminal velocities, and clumping fillingfactors. The supergiants define a very clear evolutionary sequence, in terms ofages and masses, from younger and more massive stars to older stars with lowerinitial masses. O4 supergiants cluster around the 3 Myr isochrone and are moremassive than 60 Msun, while the O5 to O7.5 stars have masses in the range 50 -40 Msun and are 4 +/- 0.3 Myr old. The surface chemical composition is typicalof evolved O supergiants (nitrogen-rich, carbon- and oxygen-poor). While theobserved ranges of carbon and nitrogen mass-fractions are compatible with thoseexpected from evolutionary models for the measured stellar masses, the N/Cratios as a function of age are inconsistent with the theoretical predictionsfor the four earliest (O4 spectral type) stars of the sample. We question theefficiency of rotational mixing as a function of age for these stars andsuggest that another mechanism may be needed to explain the observed abundancepatterns. Mass-loss rates derived with clumped-models range within a factor ofthree of the theoretical mass-loss rates. The corresponding volume-fillingfactors associated with small-scale clumping are 0.05 +/- 0.02. Clumping isfound to start close to the photosphere for all but three stars, two of whichare fast rotators.

Properties of Galactic early-type O-supergiants: A combined FUV-UV and optical analysis