Context :

Aims : We aim to constrain the properties and evolutionary status of early and mid-spectral type supergiants ( from O4 to O7.5 ) .
These posses the highest mass-loss rates among the O stars , and exhibit conspicuous wind profiles .

Methods : Using the non-LTE wind code CMFGEN Â we simultaneously analyzed the FUV-UV and optical spectral range to determine the photospheric properties and wind parameters .
We derived effective temperatures , luminosities , surface gravities , surface abundances , mass-loss rates , wind terminal velocities , and clumping filling factors .

Results : The supergiants define a very clear evolutionary sequence , in terms of ages and masses , from younger and more massive stars to older stars with lower initial masses .
O4 supergiants cluster around the 3 Myr isochrone and are more massive than 60 M _ { \odot } , while the O5 to O7.5 stars have masses in the range 50 - 40 M _ { \odot } Â and are 4 \pm 0.3 Myr old .
The surface chemical composition is typical of evolved O supergiants ( nitrogen-rich , carbon- and oxygen-poor ) .
While the observed ranges of carbon and nitrogen mass-fractions are compatible with those expected from evolutionary models for the measured stellar masses , the N/C ratios as a function of age are inconsistent with the theoretical predictions for the four earliest ( O4 spectral type ) stars of the sample .
We question the efficiency of rotational mixing as a function of age for these stars and suggest that another mechanism may be needed to explain the observed abundance patterns .
Mass-loss rates derived with clumped-models range within a factor of three of the theoretical mass-loss rates .
The corresponding volume-filling factors associated with small-scale clumping are 0.05 \pm 0.02 .
Clumping is found to start close to the photosphere for all but three stars , two of which are fast rotators .

Conclusions :