Short-period variables. VIII. Evolution and pulsation of delta Scuti stars.

Two evolutionary sequences of model stars of 1.8 and 2 Me are constructed in the post main sequence phase. The data on 6 Scuti variables are analyzed in order to place the stars in a theoretical HR diagram ( , log Te). The periods of the fundamental mode and the first overtone of the models are compared with the observed periods of the variables. The study of the vibrational stability of one model on the evolutionary sequence of 1.8 Me confirms that a opacity-mechanism can explain the pulsation of the 6 Scuti stars, as is the case with the Cepheids. The instability lies in the second helium ionization zone for the fundamental mode and the first overtone - Key words: 6 Scuti stars -evolutionary stage - mode of pulsation - opacity-mechanism