  RR Lyrae stars have been observed to improve the insight into the processes
at work in their atmospheres. Simultaneous Stroemgren-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 c_1 are
used 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 the
equations for stellar structure, additional parameters can be derived.
Stroemgren photometry and its calibration in terms of T_eff 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 T_eff and that the strength
of the Ca_ii K 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 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 gravity
calculated from T_eff, L, and the mass M,and the gravity log g_BJ. This can be
used to reassess the mass and the absolute magnitude of an individual star.The
curves derived for the pulsational velocity V_pul 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.
