  We present the results of collaborative observations of three RR Lyrae stars
(CX Lyr, NU Aur and VY CrB) which have a strong Blazhko effect. This work has
been initiated and performed in the framework of the GEOS RR Lyr Survey (Groupe
Europ\'een d'Observations Stellaires). From the measured light curves, we have
determined the times and the magnitudes at maximum. The times of maxima have
been compared to ephemerides to obtain the (O-C) values and from a period
analysis of these (O-C) values, the Blazhko period is derived. The Blazhko
periods of NU Aur (114.8 days) and VY CrB (32.3 days) are reported here for the
first time and a more accurate period for CX Lyr (68.3 days) has been obtained.
The three stars are subject to strong Blazhko effect, but this effect has
different characteristics for each of them. When we compare the variations of
magnitude at maximum and variations of (O-C) values with respect to the Blazhko
phase, these variations are either in phase, in opposition, or even in
quadrature.
