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Amplitude and phase variability are commonly found in many different types of pulsating stars. This suggests a common, presently unknown physical origin. We have examined the phenomenon in several δ Scuti stars with extensive data and find the beating of close frequencies to be responsible. This is demonstrated for the star FG Vir by testing the relationship between the observed amplitude and phase variations. Most close frequency pairs are situated near the observed or theoretically predicted frequencies of radial modes. The large number of detected close frequencies excludes the possibility of accidental fre-