  The light curves of the first overtone Pop. I Cepheids (s-Cepheids) show a
discontinuity in their phi_21 vs. P diagram, near P=3.2 day. This feature,
commonly attributed to the 2:1 resonance between the first and the fourth
overtones (omega_4=2 omega_1), is not reproduced by the hydrodynamical models.
With the goal of reexamining the resonance hypothesis, we have obtained new
CORAVEL radial velocity curves for 14 overtone Cepheids. Together with 10
objects of Krzyt et al. (1999), the combined sample covers the whole range of
overtone Cepheid periods. The velocity Fourier parameters display a strong
characteristic resonant behavior. In striking contrast to photometric ones,
they vary smoothly with the pulsation period and show no jump at 3.2 day. The
existing radiative hydrodynamical models match the velocity parameters very
well. The center of the omega_4 = 2 omega_1 resonance is estimated to occur at
P_r = 4.58+-0.04 day, i.e. at a period considerably longer than previously
assumed (3.2 day). We identify two new members of the s-Cepheid group: MY Pup
and V440 Per.
