Theoretical D_n(4000) vs. Hβ_A (upper panel) and D_n(4000) vs. Hα_A (lower panel) diagrams. The black solid line is the time evolution (from 1 Myr to 5 Gyr) as predicted by the BC03 models using solar metallicity, a Salpeter IMF, and an instantaneous burst of star formation. The dashed lines are combinations of different evolved populations (ages 700 Myr, 1 Gyr, 2 Gyr, 3 Gyr, and 5 Gyr) and a young ionizing stellar population of 6 Myr. The dotted lines represent the contribution in mass of the two stellar populations, as in Fig. 6.