The light curves of the first overtone Pop .
I Cepheids ( s-Cepheids ) show a discontinuity in their \phi _ { 21 } vs . \mathrm { P } diagram , near \mathrm { P } = 3.2 day .
This feature , commonly attributed to the 2:1 resonance between the first and the fourth overtones ( \omega _ { 4 } \approx 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 \mathrm { P } _ { \mathrm { r } } = 4.58 \pm 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 .