Change of the surface abundances of helium, lithium, beryllium and boron along evolutionary tracks of SMC composition as a function of time, expressed as a fraction of the main-sequence lifetime. Different line styles represent different initial rotational velocities (see legend). For models where the surface helium mass fraction exceeded 40%, the abundances are plotted in red, to indicate that the reference element hydrogen is depleted significantly (see also Sect. 4).