  The analysis of the multicolour photometric observations of MW Lyr, a large
modulation amplitude Blazhko variable, shows for the first time how the mean
global physical parameters vary during the Blazhko cycle. About 1-2 percent
changes in the mean radius, luminosity and surface effective temperature are
detected. The mean radius and temperature changes are in good accordance with
pulsation model results, which show that these parameters do indeed vary within
this order of magnitude if the amplitude of the pulsation changes
significantly. We interpret the phase modulation of the pulsation to be a
consequence of period changes. Its magnitude corresponds exactly what one
expects from the detected changes of the mean radius assuming that the
pulsation constant remains the same during the modulation. Our results indicate
that during the modulation the pulsation remains purely radial, and the
underlying mechanism is most probably a periodic perturbation of the stellar
luminosity with the modulation period.
