Period-colour (PC) and amplitude-colour (AC) relations at maximum, mean and
minimum light are constructed from a large grid of full amplitude hydrodynamic
models 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 metallicity.
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.Comment: 10 pages, 6 figures and 6 tables, MNRAS accepte