OGLE Cepheids have Lower Amplitudes in SMC than in LMC

We selected cepheids from the OGLE database for the Magellanic Clouds in the period range 10^{1.1} < P < 10^{1.4} days. There were 33 objects in the LMC and 35 in the SMC. We find that the median amplitude of cepheids in the LMC is 18% larger than in the SMC, a 4 sigma effect. This implies that the period - flux amplitude relation is not universal, and cannot be used to measure distances accurately.