a: projected rotational velocity vs. the nitrogen content of the stars in detached systems (i.e., subsamples 1 and 2: see Sect. 4). The circles represent the primaries whilst the triangles represent the secondaries of our sample. As a comparison, we also plot in grey the population of single giant and supergiant O-type stars in the 30 Doradus region (Grin et al. 2017). Tracks are from Brott et al. (2011a); in the top panel, they correspond to an initial mass of 25 M_⊙. The colour-bar represents the spectroscopic masses of the components. b: spectroscopic mass vs. the nitrogen content of the stars in detached systems (i.e. subsamples 1 and 2: see Sect. 4). The colour-bar represents the orbital period of the systems. c: age vs. the nitrogen content of the stars in detached systems (i.e. subsamples 1 and 2: see Sect. 4). The colour-bar represents the initial rotational velocity of the stars. d: rotational velocity vs. nitrogen content of the stars in detached systems (i.e. groups 1 and 2). The colour-bar represents the spectroscopic masses of the components.