Period distribution of Cepheids for a stellar population containing stars with two different helium abundances (Y(FG) = 0.25 and Y(SG) = 0.35), and an age difference of 20 Myr. The age of the first-generation stars is t_FG = 40 Myr (left panel) and t_FG = 120 Myr (right panel). The green line represents the period distribution expected by the double population and defines the adopted bin size (left panel: 0.025; right panel: 0.05). For clarify, the bins of the red and blue columns are drawn smaller than the adopted ones.