  The period-change behaviour of 134 RR Lyrae stars in the globular cluster
Messier 3 (M3) is investigated on the ~120-year time base of the photometric
observations. The mean period-change rates (\beta \approx 0.01 d Myr^-1) of the
subsamples of variables exhibiting the most regular behaviour are in good
agreement with theoretical expectations based on Horizontal-Branch stellar
evolution models. However, a large fraction of variables show period changes
that contradict the evolutionary expectations. Among the 134 stars studied, the
period-change behaviour of only 54 variables is regular (constant or linearly
changing), slight irregularities are superimposed on the regular variations in
23 cases and the remaining 57 stars display irregular period variations. The
light curve of ~50 per cent of the RRab stars is not stable, i.e., these
variables exhibit Blazhko modulation. The large fraction of variables with
peculiar behaviour (showing light-curve modulation and/or irregular O-C
variation) indicate that, probably, variables with regular period changes
incompatible with their evolutionary stages also could display some kind of
instability of the pulsation light curve and/or period, but the available
observations have not disclosed it yet. The temporal appearence of the Blazhko
effect in some stars, and the 70-90 years long regular changes preceded or
followed by irregular, rapid changes of the pulsation period in some cases
support this hypothesis.
  [...] Abstract truncated due to the limitations of astroph. See full abstract
in the paper.
