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tokens: large sin . To check this we included provisional results for stars with initial masses higher than 15 that have evolved into O and B giants ( Ramírez-Agudelo 2017 ; Schneider et al., in prep.). A Kuiper test of the updated distributions for groups 2 and 3 found that the probability to be drawn from the same parent distribution is still 10%. This preliminary test indicates that this possible bias can probably not explain the lack of fast rotators below log g 3.9, although we remark that the parameters for some stars still have to be confirmed, and we will revisit this in a future study. 6.1.4. Helium abundances Helium abundances for our sample were also indicated in Fig. 11 . Most of our stars have Y(He) 0.12, i.e. normal abundances within the estimated uncertainties. There were 14 stars for which we could not obtain good determinations of Y(He), and where only upper limits of 0.08 could be estimated (with the best-fitting models being 0.06 or less). As explained in Paper XIII, these low abundances could be a consequence of undetected binarity, the effects of low signal-to-noise ratio, and/or nebular contamination in the spectra. We found five stars with enhanced helium abundances (Y(He) 0.12) and log g 3.9. Two, VFTS 724 and VFTS 285, are rapid rotators with sin 370 and 600 , respectively. The rapid rotation of these two stars could explain their high helium abundances (see e.g. Maeder 1987 ; Langer 1992 ; Denissenkov 1994 ; Herrero et al. 1999 ). The other three stars (VFTS 089, 123, and 761) have intermediate and low rotation rates. Rapid rotation might still explain the He excess of these stars given that their true rotational velocities might be higher because of projection effects. Equally, their He enhancements might indicate past mass transfer from an undetected binary (e.g. Hunter et al. 2008 ), and they could be magnetic stars (see e.g. Schneider et al. 2016 ). We note that stellar winds are unlikely to be the cause for the He enrichment, as such strong winds should generate a larger surface enhancement, with wind features also seen in the spectra. 6.2. Mass-discrepancy problem As noted earlier, evolutionary masses () are typically estimated by placing a star in the H–R diagram (using results from spectral analysis) and interpolating between evolutionary tracks. Ideally, should agree with the spectroscopic mass (), calculated from , log g , and from the spectral analysis, but this is not always the case and is known as the “mass-discrepancy problem”. This discrepancy has been a challenge in the determination of masses in O-type stars since it was identified by Herrero et al. (1992) . Possible explanations have included uncertain distance estimates to Galactic stars, problems with stellar atmosphere models (e.g. underestimated gravities or the treatment of mass-loss), and problems in the evolutionary models (treatment of overshooting, rotation, and/or binary evolution). However, given a diversity of results across a range of