  The planet-star interaction is manifested in many ways. It was found out that
a close-in exoplanet causes small but measurable variability in the cores of a
few lines in the spectra of several stars which corresponds to the orbital
period of the exoplanet. Stars with and without exoplanets may have different
properties. The main goal of our study is to search for influence which
exoplanets might have on atmospheres of their host stars. Unlike the previous
studies, we do not study changes in the spectrum of a host star or differences
between stars with and without exoplanets. We aim to study a large number of
stars with exoplanets, current level of their chromospheric activity and look
for a possible correlation with the exoplanetary properties. To analyse the
chromospheric activity of stars we exploit our own (2.2m ESO/MPG telescope) and
publicly available archival spectra (Keck Observatory Archive), measure the
equivalent widths of the cores of Ca II H and K lines and use them as a tracer
of their activity. Subsequently, we search for their dependence on the orbital
parameters and mass of the exoplanet. We found a statistically significant
evidence that the equivalent width of the Ca II K line emission and log R'_{HK}
activity parameter of the host star varies with the semi-major axis and mass of
the exoplanet. Stars with T_eff <= 5500 K having exoplanets with semi-major
axis a <= 0.15 AU (P_orb <= 20 days) have a broad range of Ca II K emissions
and much stronger emission in general than stars at similar temperatures but
with higher values of semi-major axes. Ca II K emission of cold stars (T_eff <=
5500 K) with close-in exoplanets (a <= 0.15 AU) is also more pronounced for
more massive exoplanets. The overall level of the chromospheric activity of
stars may be affected by their close-in exoplanets. Stars with massive close-in
exoplanets may be more active.
