Equivalent width of the Li i 6708 Å line versus colour, with different symbols corresponding to the populations as in Fig. 9. The solid lines are isochrones calculated according to the evolutionary models of Baraffe et al. (1998, with mixing length of 1.0 pressure scale heights) at 10, 15 and 20 Myr, transformed into the observational plane as described in Sect. 3.