Illustration of the positions of VFTS 16 and 72 in the surface nitrogen enrichment versus effective temperature diagram, superimposed on the evolutionary tracks of Köhler et al. (2015) for LMC metallicity (in blue). The tracks are labelled with their initial rotational velocity, with time-steps indicated along with surface helium abundance. Since O2 dwarfs and giants typically have T_eff≳ 50 000 K one can see that for such a star to have an age of ∼1.5 Myr it would be significantly more nitrogen rich than is derived for VFTS 16, and it should be helium rich (also not inferred from the observations). For comparison we also show tracks (in red) from Choi et al. (2016), as discussed in Sect. 3.