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

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 i