  Planetary systems with close-in giant planets can experience magnetic
star-planet interactions that modify the activity levels of their host stars.
The induced activity is known to strongly depend on the magnetic moment of the
interacting planet. Therefore, such planet-induced activity should be more
readily observable in systems with planets in eccentric orbits, since those
planets are expected to rotate faster than in circular orbits. However, no
evidence of magnetic interactions has been reported in eccentric systems to
date. We intend to unveil a possible planet-induced activity in the bright ($V$
= 8.05 $\pm$ 0.03 mag) and slightly evolved star HD 118203, which hosts an
eccentric ($e$ = 0.32 $\pm$ 0.02) and close-in ($a$ = 0.0864 $\pm$ 0.0006 au)
Jupiter-sized planet. We characterized the system by modelling 56 ELODIE radial
velocities and four sectors of TESS photometry. We searched for planet-induced
and rotation-related activity signals within the TESS, ELODIE, and ASAS-SN
public data. We studied the possible origins of the variability found, analysed
its persistence and evolution, and searched for links with the eccentric
orbital motion of HD 118203 b. We found evidence of an activity signal within
the TESS data that matches the orbital period of HD 118203 b, which suggests
the existence of magnetic star-planet interactions. We did not find, however,
any additional signal that could be interpreted as the rotation of the star, so
we cannot discard stellar rotation as the source of the signal found.
Nevertheless, the evolved nature of the star and the orbital eccentricity make
the synchronous stellar rotation very unlikely. HD 118203 represents the best
evidence that magnetic star-planet interactions can be found in eccentric
systems, and it opens the door to future dedicated searches that will allow us
to better understand the interplay between close-in planets and their hosts.
