Properties of the birthline described in Sect. 4.1. Upper panel: stellar radius (upper blue curve) and the internal structure, with convective regions in dark grey, radiative regions in light grey, and iso-mass as dotted curves. Second panel: accretion of specific angular momentum (in logarithm of cm² s^-1): the black dotted line is the value given by Eq. (19) and the solid blue line indicates the angular momentum effectively accreted. Third panel: angular velocity at the surface and in the centre. The lower panel shows the ratio v_surf/v_crit (lower thick solid blue line). The filled squares connected by the thin blue line indicate the values of v_surf/v_crit reached by the models at constant mass computed from the birthline once they have converged to their MS equilibrium (see Sect. 4.2). The blue dashed line shows the values of v_surf/v_crit of the same models when they reach their minimum radius corresponding to the ZAMS. We also added the value of v_surf/v_crit these models would have at the minimum radius if angular momentum was locally conserved (blue dotted line). The grey dotted horizontal line indicates the critical limit.