THE ASYMMETRY PARAMETER M-M OF THE LIGHT CURVES OF CEPHEIDS IN THE GALAXY AND MAGELLANIC CLOUDS

The parameter M-m (that is the interval between the phases of maximum M and minimum m of the light curve) of the population I Cepheids has b een reconsidered. It is shown that the relation between M-m and P of t he first overtone Cepheids in the Magellanic Clouds, which have a peri od range between approximately 1 and approximately 6 d, is similar to that observed in the corresponding galactic Cepheids, and this support s the hypothesis of the homogeneity of the group of first overtone Cep heids in the Galaxy, identified by means of Fourier parameters of the light curves. The use of synthetic light curves allows us to understan d the trends and gaps observed in the (M-m) - P relation of fundamenta l mode Cepheids. The differences between these relations in the Galaxy and LMC and SMC are discussed. A short discussion of the P - L relati ons and some speculations on the second overtone Cepheids are presente d. Finally, stress is laid on the need of accurate light curves of Cep heids in external galaxies in order to get accurate values of the Four ier parameters and a better knowledge of the position of the resonance centers. These centers are of fundamental importance for the verifica tion of the theories on stellar interior and evolution.