Orbital period distribution of stars residing inside the Cepheid instability strip at solar metallicity, Z = 0.02, according to our population synthesis models. (Left) Binary systems for the entire period range, systems with an orbital period longer than 11 yr are not plotted. (Right) Systems with orbital periods shorter than 1 yr. The dotted histogram represents all systems evolving across the instability strip while the solid histogram represents those systems identified as Cepheids, i.e., with no significant mass transfer from the primary to secondary.