The two period-luminosity relations for population I Cepheids

We summarize the evidence that most Population I Cepheids with periods less than approximately 8 days pulsate in the first overtone mode. Fundamental model and first overtone pulsators must follow different period-luminosity (P - L) relations. We demonstrate these different relations for different stellar systems, especially for the calibrating Cepheids in clusters and for Cepheids in the Large Magellanic Cloud (LMC), the Small Magellanic Cloud (SMC), M31, and IC 4182.