  Pulsators on and near the main sequence show period and amplitude changes
that are too large to be the product of stellar evolution. The multiperiodic
Delta Scuti stars are well suited to study these changes. This requires a very
large amount of photometric data covering years and decades as well as mode
identifications. We have examined over 800 nights of high-precision photometry
of the multiperiodic pulsator 4 CVn obtained from 1966 through 2012. Because
most of the data were obtained in adjacent observing seasons, it is possible to
derive very accurate period values for a number of the excited pulsation modes
and to study their systematic changes from 1974 to 2012. Most pulsation modes
show systematic significant period and amplitude changes on a timescale of
decades. For the well-studied modes, around 1986 a general reversal of the
directions of both the positive and negative period changes occurred.
Furthermore, the period changes between the different modes are strongly
correlated, although they differ in size and sign. For the modes with known
values of the spherical degree and azimuthal order, we find a correlation
between the direction of the period changes and the identified azimuthal order,
m. The associated amplitude changes generally have similar timescales of years
or decades, but show little systematic or correlated behavior from mode to
mode. A natural explanation for the opposite behavior of the prograde and
retrograde modes is that their period changes are driven by a changing rotation
profile. The changes in the rotation profile could in turn be driven by
processes, perhaps the pulsations themselves, that redistribute angular
momentum within the star. In general, different modes have different rotation
kernels, so this will produce period shifts of varying magnitude for different
modes.
