Period-colour and amplitude-colour relations in classical Cepheid variables V: The Small Magellanic Cloud Cepheid models

Period-colour (PC) and amplitude-colour (AC) relations at maximum, mean and minimum light are constructed from a large grid of full amplitude hydrodynamic mod-els of Cepheids with a composition appropriate for the SMC (Small Magellanic Cloud). We compare these theoretical relations with those from observations. The theoretical relations are in general good agreement with their observational counterparts though there exist some discrepancy for short period (log[P] < 1) Cepheids. We outline a physical mechanism which can, in principle, be one factor to explain the observed PC/AC relations for the long and short period Cepheids in the Galaxy, LMC and SMC. Our explanation relies on the hydrogen ionization front-photosphere interaction and the way this interaction changes with pulsation period, pulsation phase and metal-licity. Since the PC relation is connected with the period-luminosity (PL) relation, it is postulated that such a mechanism can also explain the observed properties of the PL relation in these three galaxies. Key words: Cepheids { Stars: fundamental parameters