  In most Delta Scuti stars, the measured period changes are considerably
larger than those expected from stellar evolution. In order to study these
period and amplitude changes, a few selected stars are measured photometrically
for hundreds of nights with a dedicated telescope. These measurements cover
several years or decades. The Delta Scuti stars provide more information (than
classical pulsators) since a number of simultaneously excited radial and
nonradial modes can be studied. The present results indicate the presence of at
least two effects: beating of independent modes with close frequencies and
stellar cycles. For period and amplitude changes with time scales less than one
year, we confirm the beating hypothesis in three stars. This is shown by the
correctly correlated relationship between amplitude and phase changes as well
as the repetitions of these cycles. However, the observed period variations
with longer time scales are not due to simple beating between two close
frequencies. For the star 4 CVn we can derive accurate annual frequency values
for at least seven radial and nonradial modes. The annual phases are in
excellent agreement with predictions from nearby years, thereby confirming the
values and their observed long-term changes. For prograde and retrograde modes,
the period variations are of identical size, but with opposite signs. The
radial mode shows no (or little) changes. Furthermore, all period variations
show a reversal around 1990. These results suggest long-term, regular cycles
affecting individual modes differently with some common systematic behavior.
