The P-C and P-L relations of the fundamental-mode (P0) Cepheids of IC 1613. The full lines are the corresponding relations of SMC. Forced linear fits over the whole period interval are shown as dashed lines (their equations are given at the bottom of each panel). The agreement between IC 1613 and SMC is striking. Cepheids excluded by the original authors (triangles) or by ourselves (diamonds) are shown as open symbols.