  We analyze the historical light curve of the symbiotic star YY Her, from 1890
up to December 2005. A secular declining trend is detected, at a rate of ~.01
magn in 1000 d, suggesting that the system could belong to the sub-class of
symbiotic novae. Several outburst events are superposed on this slow decline.
Three independent periodicities are identified in the light curve. A
quasi-periodicity of 4650.7 d is detected for the outburst occurrence. We
suggest that it is a signature of a solar-type magnetic dynamo cycle in the
giant component. A period of 593.2 d modulates the quiescent light curve and it
is identified as the binary period of the system. During outburst events the
system shows a stable periodic oscillation of 551.4 d. We suggest that it is
the rotation period of the giant.The secondary minima detected at some epochs
of quiescence are probably due to dark spots on the surface of the rotating
giant.
  The difference between the frequencies of these two last periods is the
frequency of a tidal wave in the outer layers of the giant. A period which is a
beat between the magnetic cycle and the tidal wave period is also apparent in
the light curve. YY Her is a third symbiotic system exhibiting these cycles in
their light curve, suggesting that a magnetic dynamo process is prevalent in
the giant components of symbiotic stars, playing an important role in the
outburst mechanism of some of these systems.
