Cumulative distribution of orbital periods of ProCeps (taken at t₀; black solid line) and BiCeps (taken at t_He,end; dashed lines). The orbital period distribution of BiCeps is shown separately for Cepheids in the lower (magenta dashed line) and more massive clusters (cyan dashed line), in the whole cluster population (yellow dashed line), in the field originating from the whole cluster population (black dashed line), and in the control model (brown dashed line). The orbital period distribution of Cepheids with a compact companion is indicated by the black dotted line. The distributions are normalised to the total number of BiCeps in each group. There are almost no BiCeps with log₁₀(P[days]) ≲ 2 because of coalescence with their companion before the onset of core helium burning. The vertical dotted line represents the upper limit on the initial orbital periods (log₁₀(P[days]) ≲ 6.7). All BiCeps with an orbital period longer than that formed due to dynamical encounters in clusters.