Hysteresis of atmospheric parameters of 12 RR Lyrae stars

Aims. RR Lyrae stars have been observed to improve the insight into processes at work in their atmospheres. Methods. Simultaneous Strömgren-photometry allows to obtain a rapid sequence of measurements in which photometric indices are unaffected by non-optimum observing conditions. The indices y, b − y, and c1 are used with an established calibration to derive Teff and to determine the gravity, log gBJ from the Balmer jump, throughout the pulsation cycle. By employing the equations for stellar structure, additional parameters can be derived. Results. Strömgren photometry and its calibration in terms of Teff and log g can be used to determine the run of R and the atmosphere pulsation velocity. We find that the Balmer-line strengths are correlated with Teff and that the strength of the Ca iiK line correlates well with the radius of the star and thus the pulsation-dependent density of the atmosphere. The density in the stellar atmosphere fluctuates as indicated by the changes in the gravity log gBJ, derived from c1, between 2.3 and 4.5 dex. Also the Strömgren metal index, m1, fluctuates. We find a disagreement between log g(T, L,M), the gravity calculated from Teff, L, and the mass M, and the gravity log gBJ. This can be used to reassess the mass and the absolute magnitude of an individual star. The curves derived for the pulsational velocity Vpul differ from curves obtained from spectra needed to apply the Baade-Wesselink method; we think these differences are due to phase dependent differences in the optical depth levels sampled in continuum photometry and in spectroscopy. We find an atmospheric oscillation in these fundamental mode RR Lyrae stars of periodicity P/7