Period versus age for early fast rotator pMS stars of the ONC. Their positions in the period-age plane are compared with models assuming evolution with constant angular momentum (Landin et al. 2006). Masses in the interval 0.2 <M/M_⊙< 0.3 correspond to °, 0.3 <M/M_⊙< 0.4 to ▪, and all the remaining to ×. Solid lines represent models with J_in = J_kaw, while dashed lines stand for models with J_in = 3J_kaw for M = 0.2, 0.3 and 0.4 M_⊙. Figure adapted from Landin et al. (2006).