  The activity levels of stars are influenced by several stellar properties,
such as stellar rotation, spectral type and the presence of stellar companions.
In analogy to binaries, planetary companions are also thought to be able to
cause higher activity levels in their host stars, although at lower levels.
Especially in X-rays, such influences are hard to detect because coronae of
cool stars exhibit a considerable amount of intrinsic variability. Recently, a
correlation between the mass of close-in exoplanets and their host star's X-ray
luminosity has been detected, based on archival X-ray data from the ROSAT
All-Sky Survey. This finding has been interpreted as evidence for Star-Planet
Interactions. We show in our analysis that this correlation is caused by
selection effects due to the flux limit of the X-ray data used and due to the
intrinsic planet detectability of the radial velocity method, and thus does not
trace possible planet-induced effects. We also show that the correlation is not
present in a corresponding complete sample derived from combined XMM-Newton and
ROSAT data.
