Period-colour and amplitude-colour relations for MACHO project Large Magellanic Cloud RR Lyrae stars

In this Letter, we analyse period–colour and amplitude–colour relations at minimum, mean and maximum V-band light for 6391 RRab stars in the Large Magellanic Cloud obtained by the MACHO project. Specifically, we find that colour and amplitude are nearly independent of period at minimum light, but that there exists a definite relation between period and colour and amplitude and colour at maximum light. These two properties are consistent with earlier work suggesting that these observed properties are, at least to some extent, explained by the application of the Stefan–Boltzmann law and the interaction of the photosphere and hydrogen ionization front at minimum light. When we examine the slope of the period–colour relation as a function of phase, we find that the slope varies significantly with phase and is small for a wide range of phases around minimum light. This suggests that another factor that needs to be considered when trying to understand RR Lyrae observed properties is their behaviour at different phases during a pulsation cycle.