EW_Hδ in absorption as function of burst age in two stellar populations of different star formation histories according to Bruzual & Charlot (2003). No nebular emission is included in the model. The upper diagram a) shows the evolution in a population that has experienced a short burst and in b) the SFR has been constant for 100 Myr before being abruptly shut down. The model results for three different metallicities, Z = 0.005 (dashed-dot blue), Z = 0.02 (solid green) and Z = 0.05 (dashed red) are shown. Our selection criterion for a post-starburst galaxy (EW_Hδ<−6 Å) is indicated with the horizontal yellow line.