Diagrams showing dependence between EW and metallicity for the 1501 and 1719 Å absorption line indexes that we adopt in this paper according to Starburst99 models. All the data points are derived assuming a constant SFH for 100 Myr and Kroupa IMF. The shaded gray region around the main relations represents the variation of EW with the IMF (Kroupa-Salpeter) and with the ages of the stellar population chosen (50 Myr–2 Gyr), as described in the text. The final metallicity calibrations are based on a third-order polynomial fit to the data points, and are shown with a continuous blue line in each panel. Their explicit forms are given in the text in Eqs. (2) and (3).