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Aims: Recent observations have challenged our understanding of rotationalmixing in massive stars by revealing a population of fast-rotating objects withapparently normal surface nitrogen abundances. However, several questions havearisen because of a number of issues, which have rendered a reinvestigationnecessary; these issues include the presence of numerous upper limits for thenitrogen abundance, unknown multiplicity status, and a mix of stars withdifferent physical properties, such as their mass and evolutionary state, whichare known to control the amount of rotational mixing. Methods: We havecarefully selected a large sample of bright, fast-rotating early-type stars ofour Galaxy (40 objects with spectral types between B0.5 and O4). Theirhigh-quality, high-resolution optical spectra were then analysed with thestellar atmosphere modelling codes DETAIL/SURFACE or CMFGEN, depending on thetemperature of the target. Several internal and external checks were performedto validate our methods; notably, we compared our results with literature datafor some well-known objects, studied the effect of gravity darkening, orconfronted the results provided by the two codes for stars amenable to bothanalyses. Furthermore, we studied the radial velocities of the stars to assesstheir binarity. Results: This first part of our study presents our methods andprovides the derived stellar parameters, He, CNO abundances, and themultiplicity status of every star of the sample. It is the first time that Heand CNO abundances of such a large number of Galactic massive fast rotators aredetermined in a homogeneous way.

Chemical abundances of fast-rotating massive stars. I. Description of the methods and individual results