The importance of colour-colour loops in the determination of the physical parameters of pulsating variables

Attention is drawn to the importance of studying the surface brightness of pulsating stars as a function of two colors, in order to disentangle the contributions of temperature and gravity variations to the observed curve from that due to the simultaneous variation of the emitting surface of the star. The area of the observed color-color loops obtained with different colors is compared with the area delta B of the surface-brightness color loop, inferred from calibrations. The existence of the color-color loops and the dependence of the loop area on the pulsation period, very similar to that of delta B, is easily explained, under the assumption of quasi-static approximation, by the existence of a loop in the plane of effective temperature and effective gravity, the area of which varies with the period. The consistency and adequacy of the whole picture on which the CORS method (used to determine the radius of classical Cepheids) is based are strongly supported by the experimental evidence.