  The shape of the radial velocity and light curves of 24 long-period (30 < P <
134 days) 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 sample of stars; however, evident changes of the shape can be seen in
Cepheids with period between 90 and 134 d. The Fourier parameter-period
diagrams 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 the study of stellar structure and
evolution in relatively far galaxies is emphasized.
