Abundance ratios as a function of the iron abundance. Left panels: same data as in Fig. 2, but comparing the [O/Fe] (top) and [S/Fe] (bottom) ratios for our samples of classical Cepheids and open clusters. The bottom panel also shows the abundance ratios for the open clusters (Trumpler 5 and Trumpler 20) provided by Lucertini et al. (2023). Right panels: [O/Fe] and [S/Fe] ratios for Galactic Cepheids compared with thin-and thick-disk stars provided by the GES (DR5.0) collaboration and with thin-disk, thick-disk, and bulge field stars provided by Lucertini et al. (2022, L22a).