THE STRUCTURE OF THE INSTABILITY STRIP AND MODE-IDENTIFICATION FOR BETA-CEP STARS IN 3 YOUNG OPEN CLUSTERS

We use a grid of stellar models to calculate isochrones. These are use d to determine the ages of NGC 3293, NGC 4755 and NGC 6231. Using a no n-adiabatic pulsation code, we compute the instability strip for beta Cep variables. We find that there is a correlation between the frequen cies of the excited modes and the mass for stars of a given age. A cal ibration is presented which allows age determination of beta Cep stars in clusters with greater accuracy than isochrone fitting. The strong dependence of the frequency-mass relationship on age explains the temp erature difference between the instability strips in NGC 3293 and NGC 4755. The second part of the paper examines the question of mode ident ification for beta Cep stars in NGC 3293 and NGC 4755. We present new uvby observations for these stars and use the dependence of the amplit ude on wavelength to estimate the modes. We are able to isolate a few radial pulsators in the two clusters and to deduce their masses, effec tive temperatures and luminosities.