Period-colour and amplitude-colour relations in classical Cepheid variables

In this paper we analyse the behaviour of Galactic, Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) Cepheids in terms of period-colour (PC) and amplitude-colour (AC) diagrams at the phases of maximum, mean and minimum light. We find very different behaviour between Galactic and Magellanic Cloud Cepheids. Motivated by the recent report by Tammann et al., of a break in the LMC PC relations at 10 d, we use the statistical F-test to examine the PC relations at mean light in these three galaxies. The results of the F-test support the existence of the break occurring in the LMC PC(mean) relation, but not in the Galactic or SMC PC(mean) relation. Furthermore, the LMC Cepheids also show a break at minimum light, which is not seen in the Galactic and SMC Cepheids. We further discuss the effect on the period-luminosity relations in the LMC due to the break in the PC(mean) relation.