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RR Lyrae stars have been observed to improve the insight into the processesat work in their atmospheres. Simultaneous Stroemgren-photometry allows toobtain a rapid sequence of measurements in which photometric indices areunaffected by non-optimum observing conditions. The indices y, b-y, and c_1 areused with an established calibration to derive T_eff and to derive the gravity,log g_BJ from the Balmer jump, throughout the pulsation cycle. By employing theequations for stellar structure, additional parameters can be derived.Stroemgren photometry and its calibration in terms of T_eff and log g can beused to determine the run of R and the atmosphere pulsation velocity. We findthat the Balmer-line strengths are correlated with T_eff and that the strengthof the Ca_ii K line correlates well with the radius of the star and thus thepulsation-dependent density of the atmosphere. The density in the stellaratmosphere fluctuates as indicated by the changes in the gravity log g_BJ,derived from c_1, between 2.3 and 4.5 dex. Also the Stroemgren metal index,m_1, fluctuates. We find a disagreement between log g(T,L,M), the gravitycalculated from T_eff, L, and the mass M,and the gravity log g_BJ. This can beused to reassess the mass and the absolute magnitude of an individual star.Thecurves derived for the pulsational velocity V_pul differ from curves obtainedfrom spectra needed to apply the Baade-Wesselink method; we think thesedifferences are due to phase dependent differences in the optical depth levelssampled in continuum photometry and in spectroscopy. We find an atmosphericoscillation in these fundamental mode RR Lyrae stars of periodicity P/7.

Hysteresis of atmospheric parameters of 12 RR Lyrae stars based on multichannel simultaneous Stroemgren photometry