THE POSSIBLE EFFECTS OF AN UNUSUAL RESONANCE IN VERY LONG-PERIOD CEPHEIDS

The shape of the radial velocity and light curves of 24 long-period (3 0 less than or equal to P less than or equal to 134 d) Cepheids in the Magellanic Clouds shows a progression with the period. The sequences of the radial velocity and light curves are based only on a small samp le of stars; however, evident changes of the shape can be seen in Ceph eids with period between 90 and 134 d. The Fourier parameter-period di agrams for the radial velocity curves show trends which remind in part those of Cepheids with period near 10 d. The plausible interpretation is a resonance, probably P-0/P-1 = 2 between the fundamental and the first overtone mode. The possible importance of this phenomenon for th e study of stellar structure and evolution in relatively far galaxies is emphasized(1).