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CONTEXT. Exoplanet searches have demonstrated that giant planets arepreferentially found around metal-rich stars and that their fraction increaseswith the stellar mass. AIMS. During the past six years, we have conducted aradial velocity follow-up program of 166 giant stars, to detect substellarcompanions, and characterizing their orbital properties. Using thisinformation, we aim to study the role of the stellar evolution in the orbitalparameters of the companions, and to unveil possible correlations between thestellar properties and the occurrence rate of giant planets. METHODS. UsingFEROS and CHIRON spectra, we have computed precision radial velocities and wehave derived atmospheric and physical parameters for all of our targets.Additionally, velocities computed from UCLES spectra are presented here. Bystudying the periodic radial velocity signals, we have detected the presence ofseveral substellar companions. RESULTS. We present four new planetary systemsaround the giant stars HIP8541, HIP74890, HIP84056 and HIP95124. Additionally,we find that giant planets are more frequent around metal-rich stars, reachinga peak in the detection of $f$ = 16.7$^{+15.5}_{-5.9}$% around stars with[Fe/H] $\sim$ 0.35 dex. Similarly, we observe a positive correlation of theplanet occurrence rate with the stellar mass, between M$_\star$ $\sim$ 1.0 -2.1M$_\odot$, with a maximum of $f$ = 13.0$^{+10.1}_{-4.2}$%, at M$_\star$ = 2.1M$_\odot$. CONCLUSIONS. We conclude that giant planets are preferentiallyformed around metal-rich stars. Also, we conclude that they are moreefficiently formed around more massive stars, in the mass range of M$_\star$$\sim$ 1.0 - 2.1 M$_\odot$. These observational results confirm previousfindings for solar-type and post-MS hosting stars, and provide further supportto the core-accretion formation model.

Four new planets around giant stars and the mass-metallicity correlation of planet-hosting stars