  Although timing variations in close binary systems have been studied for a
long time, their underlying causes are still unclear. A possible explanation is
the so-called Applegate mechanism, where a strong, variable magnetic field can
periodically change the gravitational quadrupole moment of a stellar component,
thus causing observable period changes. One of the systems exhibiting such
strong orbital variations is the RS CVn binary HR 1099, whose activity cycle
has been studied by various authors via photospheric and chromospheric activity
indicators, resulting in contradicting periods. We aim at independently
determining the magnetic activity cycle of HR 1099 using archival X-ray data to
allow for a comparison to orbital period variations. Archival X-ray data from
80 different observations of HR 1099 acquired with 12 different X-ray
facilities and covering almost four decades were used to determine X-ray fluxes
in the energy range of 2-10 keV via spectral fitting and flux conversion. Via
the Lomb-Scargle periodogram we analyze the resulting long-term X-ray light
curve to search for periodicities. We do not detect any statistically
significant periodicities within the X-ray data. An analysis of optical data of
HR 1099 shows that the derivation of such periods is strongly dependent on the
time coverage of available data, since the observed optical variations strongly
deviate from a pure sine wave. We argue that this offers an explanation as to
why other authors derive such a wide range of activity cycle periods based on
optical data. We conclude that our analysis constitutes the longest stellar
X-ray activity light curve acquired to date, yet the still rather sparse
sampling of the X-ray data, along with stochastic flaring activity, does not
allow for the independent determination of an X-ray activity cycle.
