  We have reanalyzed the long-term optical light curve (LC) of the symbiotic
star Z Andromedae, covering 112--yr of mostly visual observations. Two strictly
periodic and one quasi-periodic cycles can be identified in this LC. A P1=7550
d quasi periodicity characterizes the repetition time of the outburst episodes
of this symbiotic star. Six such events have been recorded so far. During
quiescence states of the system, i.e. in time intervals between outbursts, the
LC is clearly modulated by a stable coherent period of P2=759.1 d. This is the
well known orbital period of the Z And binary system that have been measured
also spectroscopically. A third coherent period of P3=658.4 d is modulating the
intense fluctuations in the optical brightness of the system during outbursts.
We attribute the trigger of the outbursts phenomenon and the clock that drives
it, to a solar type magnetic dynamo cycle that operates in the convection and
the outer layers of the giant star of the system. We suggest that the intense
surface activity of the giant star during maximum phases of its magnetic cycle
is especially enhanced in one or two antipode regions, fixed in the atmosphere
of the star and rotating with it. Such spots could be active regions around the
North and the South poles of a general magnetic dipole field of the star. The
P3 periodicity is half the beat of the binary orbital period of the system and
the spin period of the giant. The latter is then either 482 or 1790 d. If only
one pole is active on the surface of the giant, P3 is the beat period itself,
and the spin period is 352 d. It could also be 5000 d if the giant is rotating
in retrograde direction. We briefly compare these findings in the LC of Z And
to similar modulations that were identified in the LC of two other prototype
symbiotics, BF Cyg and YY Her.
