Damasso, M.M.DamassoEsposito, M.M.EspositoNascimbeni, V.V.NascimbeniDesidera, S.S.DesideraBonomo, A. S.A. S.BonomoBieryla, A.A.BierylaMalavolta, L.L.MalavoltaBiazzo, K.K.BiazzoSozzetti, A.A.SozzettiCovino, E.E.CovinoLatham, D. W.D. W.LathamGandolfi, D.D.GandolfiRainer, M.M.RainerPetrovich, C.C.PetrovichCollins, K. A.K. A.CollinsBoccato, C.C.BoccatoClaudi, R. U.R. U.ClaudiCosentino, R.R.CosentinoGratton, R.R.GrattonLanza, A. F.A. F.LanzaMaggio, A.A.MaggioMicela, G.G.MicelaMolinari, E.E.MolinariPagano, I.I.PaganoPiotto, G.G.PiottoPoretti, E.E.PorettiSmareglia, R.R.SmaregliaDi Fabrizio, L.L.Di FabrizioGiacobbe, P.P.GiacobbeGomez-Jimenez, M.M.Gomez-JimenezMurabito, S.S.MurabitoMolinaro, M.M.MolinaroAffer, L.L.AfferBarbieri, M.M.BarbieriBedin, L. R.L. R.BedinBenatti, S.S.BenattiBorsa, F.F.BorsaMaldonado, J.J.MaldonadoMancini, L.L.ManciniScandariato, G.G.ScandariatoSouthworth, J.J.SouthworthSanchez, R. ZanmarR. ZanmarSanchez2025-12-302025-12-3020151432-0746https://hdl.handle.net/20.500.12740/24096Aims. For more than 1.5 years we spectroscopically monitored the star KELT-6 (BD+31 2447), which is known to host the transiting hot-Saturn KELT-6 b, because a previously observed long-term trend in radial velocity time series suggested that there is an outer companion. Methods. We collected a total of 93 new spectra with the HARPS-N and TRES spectrographs. A spectroscopic transit of KELT-6 b was observed with HARPS-N, and simultaneous photometry was obtained with the IAC-80 telescope. Results. We proved the existence of an outer planet with a mininum mass M(p)sin i = 3.71 +/- 0.21 M-Jup and a moderately eccentric orbit (e = 0.21(-0.036)(+0.039)) of period P similar to 3.5 years. We improved the orbital solution of KELT-6 b and obtained the first measurement of the Rossiter-McLaughlin effect, showing that the planet has a likely circular, prograde, and slightly misaligned orbit with a projected spin-orbit angle of lambda = - 36 +/- 11 degrees. We improved the KELT-6 b transit ephemeris from photometry and provide new measurements of the stellar parameters. KELT-6 appears as an interesting case for studying the formation and evolution of multi-planet systems.Acceso Abiertostars: individual: KELT-6planetary systemstechniques: radial velocitiestechniques: photometrichttp://dx.doi.org/10.1051/0004-6361/201526995