Evolutionary tracks in the HRD. The colours along the tracks indicate the minimum magnetic field (assumed to be aligned with the rotation axis and dipolar) able to couple the stellar wind with the stellar surface and thus to exert a torque. Higher magnetic fields than the minimum value indicated here would reduce the angular momentum content of the star. The minimum value is obtained by imposing that the parameter η defined in Ud-Doula et al. (2009) is equal to one. Left panel: main-sequence and RSG phases. Right panel: WR phase.