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Context. Identification of planetary companions of giant stars is madedifficult because of the astrophysical noise, that may produce radial velocity(RV) variations similar to those induced by a companion. On the other hand anystellar signal is wavelength dependent, while signals due to a companion areachromatic. Aims. Our goal is to determine the origin of the Doppler periodicvariations observed in the thick disk K giant star TYC 4282-605-1 by HARPS-N atthe Telescopio Nazionale Galileo (TNG) and verify if they can be due to thepresence of a substellar companion. Methods. Several methods have been used toexclude the stellar origin of the observed signal including detailed analysisof activity indicators and bisector and the analysis of the photometric lightcurve. Finally we have conducted an observational campaign to monitor the nearinfrared (NIR) RV with GIANO at the TNG in order to verify whether the NIRamplitude variations are comparable with those observed in the visible.Results. Both optical and NIR RVs show consistent variations with a period at101 days and similar amplitude, pointing to the presence of a companionorbiting the target. The main orbital properties obtained for our giant starwith a derived mass of M=0.97+-0.03M_sun areM_Psini=10.78+-0.12MJ;P=101.54+-0.05days;e=0.28+-0.01 and a=0.422+-0.009AU. Thechemical analysis shows a significant enrichment in the abundance of Nai, Mgi,Ali and S i while the rest of analyzed elements are consistent with the solarvalue demonstrating that the chemical composition corresponds with an old Kgiant (age = 10.1 Gyr) belonging to local thick disk. Conclusions. We concludethat the substellar companion hypothesis for this K giant is the bestexplanation for the observed periodic RV variation. This study also shows thehigh potential of multi-wavelength RV observations for the validation of planetcandidates.

astronomy and astrophysics

The GAPS Programme with HARPS-N at TNG XV. A substellar companion around  a K giant star identified with quasi-simultaneous HARPS-N and GIANO  measurements