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Amplitude and period variations have been detected in almost all types ofpulsating stars. Many of these modulations are nearly periodic. In RR Lyraestars the phenomenon in known as the Blazhko Effect. Because of the observedsimilarities, we propose to extend the definition to other groups. Weillustrate the Blazhko Effect with examples from Cepheids, RR Lyrae stars,White Dwarfs, sdB stars and delta Scuti variables. For delta Scuti stars thepresent results indicate the presence of at least two effects: beating ofindependent modes with close frequencies and stellar cycles. For period andamplitude changes with timescales less than one year, the beating hypothesisexplains the observed modulations very well. This has been shown by thecorrectly correlated relationship between amplitude and phase changes as wellas the repetitions of these cycles. However, the observed period variationswith longer timescales cannot be due to simple beating between two closefrequencies. For the star AI CVn (= 4 CVn), we can derive accurate annualfrequency values for a number of radial and nonradial modes from 1974 to 2009.For prograde and retrograde modes, the frequency variations are of similarsize, but with opposite signs. The radial mode shows no (or little) changes.Fur- thermore, all frequency variations show a reversal around 1990. Theseresults are consistent with long-term cycles affecting individual modesdifferently with some common systematic behavior. The observed change in thesize of rotational splitting is interpreted as a change in the differentialrotation during the stellar cycle.

The Blazhko Effect in Delta Scuti and other Groups of Pulsating Stars