Variation of the equivalent width of the Hα (upper panel) and Hβ (middle panel) Balmer absorption lines, in Å, predicted by the MILES population synthesis models, as a function of the delayed exponentially declining time scale τ for a Sandage law (left) and of the stellar mass for the Boissier & Prantzos (2000) chemo-spectrophotometric models of galaxy secular evolution (right). The red line is for solar metallicities, the blue one for Z = 1 / 4   Z _⊙ . The Balmer absorption lines are measured for five different τ (2, 5, 8, 10, 20 Gyr) and stellar masses (log M_star = 8.89, 9.92, 10.52, 10.94, 11.25 M _⊙ ). The horizontal solid and dotted lines give the mean value  ±  the standard deviation of EW Hα_abs (2.8  ±  0.38 Å) and EW Hβ_abs (4.4  ±  0.63 Å) of Moustakas & Kennicutt (2006) for their sample of galaxies. The lower panel gives the expected distribution of τ and M_star for the late-type HRS galaxies analysed in this work. The values of τ are estimated using the τ – L_H luminosity relation given in Gavazzi et al. (2002a,b): log τ =  − 0.149 × log    L_H + 2.221. The blue and red histograms give the observed distribution of HRS galaxies with metallicity 12 + log(O/H)  ≤  8.5 and 12 + log(O/H)  >  8.5, respectively. The right histogram in the middle row shows the distribution of the observed underlying Balmer absorption at Hβ for the whole s ample (black) and for metal poor (blue) and metal rich (red) galaxies.