The separation of s-Cepheids from classical Cepheids and a new definition of the class.

Fourier decomposition has been applied to a sample of 184 classical and S-Cepheids with P less than 8 d and a careful evaluation of errors in the determination of the parameters has been made. The S-Cepheids stars are redefined by the authors as Population I Cepheids that do not follow the Hertzsprung progression, but have a progression of their own. In the phi(21)-P plane, the S- and classical Cepheids are characterized by two sequences well separated for P less than 5.5 d. In the period range P between 3d and 5.5 d, two different progressions are also present in the phi(31)-P plane while a discriminating value R(21) = 0.20 can be seen in the R(21)-P plane. The first overtone pulsation seems to be well established for S-Cepheids with P less then 3.2 d; it is probable for all the stars of the redefined subclass. A discontinuity is clearly visible at about 3 d in the S-Cepheid sequence in the phi(21)-P plane; it is interpreted as a resonance effect. An apparent decrease in the number of stars is present in the classical sequence for P less than 3 d.