  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 Delta 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 frequency agreements.
