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Pulsators on and near the main sequence show period and amplitude changesthat are too large to be the product of stellar evolution. The multiperiodicDelta Scuti stars are well suited to study these changes. This requires a verylarge amount of photometric data covering years and decades as well as modeidentifications. We have examined over 800 nights of high-precision photometryof the multiperiodic pulsator 4 CVn obtained from 1966 through 2012. Becausemost of the data were obtained in adjacent observing seasons, it is possible toderive very accurate period values for a number of the excited pulsation modesand to study their systematic changes from 1974 to 2012. Most pulsation modesshow systematic significant period and amplitude changes on a timescale ofdecades. For the well-studied modes, around 1986 a general reversal of thedirections of both the positive and negative period changes occurred.Furthermore, the period changes between the different modes are stronglycorrelated, although they differ in size and sign. For the modes with knownvalues of the spherical degree and azimuthal order, we find a correlationbetween the direction of the period changes and the identified azimuthal order,m. The associated amplitude changes generally have similar timescales of yearsor decades, but show little systematic or correlated behavior from mode tomode. A natural explanation for the opposite behavior of the prograde andretrograde modes is that their period changes are driven by a changing rotationprofile. The changes in the rotation profile could in turn be driven byprocesses, perhaps the pulsations themselves, that redistribute angularmomentum within the star. In general, different modes have different rotationkernels, so this will produce period shifts of varying magnitude for differentmodes.

Nonradial and radial period changes of the Delta Scuti star 4 CVn II. Systematic behavior over 40 years