The theoretical period-luminosity relation for Cepheids was deduced from stellar evolution and pulsation computations. Questions concerning the metallicity dependence of the relation as well as the constancy of the pulsation "" constant ÏÏ Q in the instability strip were addressed. We found no signiÐcant metallicity dependence of either the slope or the zero point of the period-luminosity relation. The Q-value, however, varies across the instability strip in e †ective temperature as well as in luminosity ; its metallicity dependence, on the other hand, is marginal.

Abstract. Stability properties are presented of dipole and quadrupole nonradial oscillation modes of model stars that experienced a late helium shell flash on their way to the white-dwarf cooling domain. The computed instability domains are compared with the observed hot variable central stars of planetary nebulae and the GW Vir pulsators.

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Consistently with a trend observed in recent past triennial reports, progress in the theory of stellar atmospheres continues to be made in two different directions: 1) the traditional areas of continuum and line radiation transfer, line blanketing, atomic physics and atmospheric structures controlled by the joint conditions of radiative and hydrostatic equilibrium, and 2) in areas of emerging interest such as oscillations, velocity fields, winds and mass loss, chromospheres, coronae and magnetic phenomena. Particular attention is devoted also to the accurate determination of stellar elemental abundances and their implications for stellar interior structure and evolution as well as for the chemical enrichment of the Galaxy.