Grouffal, S.S.GrouffalSanterne, A.A.SanterneBourrier, V.V.BourrierKunovac, V.V.KunovacDressing, C.C.DressingAkinsanmi, B.B.AkinsanmiArmstrong, C.C.ArmstrongBaliwal, S.S.BaliwalBalsalobre-Ruza, O.O.Balsalobre-RuzaBarros, S. C.C.S. C.C.BarrosBayliss, D.D.BaylissCrossfield, I. J.M.I. J.M.CrossfieldDemangeon, O.O.DemangeonDumusque, X.X.DumusqueGiacalone, S.S.GiacaloneHarada, C. K.C. K.HaradaIsaacson, H.H.IsaacsonKellermann, H.H.KellermannLillo-Box, J.J.Lillo-BoxLlama, J.J.LlamaMortier, A.A.MortierPalle, E.E.PalleRajpurohit, A. S.A. S.RajpurohitRice, M.M.RiceSantos, N. C.N. C.SantosSeidel, J. V.J. V.SeidelSharma, R.R.SharmaSousa, S. G.S. G.SousaThomas, L.L.ThomasTurtelboom, E. V.E. V.TurtelboomUdry, S.S.UdryWheatley, P. J.P. J.Wheatley2026-01-122026-01-122025-09-0110.1051/0004-6361/2025554872-s2.0-105016333757http://repository.iitgn.ac.in/handle/IITG2025/33832The obliquity between the stellar spin axis and the planetary orbit, detected via the Rossiter-McLaughlin (RM) effect, is a tracer of the formation history of planetary systems. While obliquity measurements have been extensively applied to hot Jupiters and short-period planets, they remain rare for cold and long-period planets due to observational challenges, particularly their long transit durations. We report the detection of the RM effect for the 19-hour transit of HIP 41378 f, a temperate giant planet on a 542-day orbit, observed through a worldwide spectroscopic campaign. We measured a slight projected obliquity of 21 ± 8 and a significant 3D spin-orbit angle of 52 ± 6, based on the measurement of the stellar rotation period. HIP 41378 f is part a transiting system of five planets with planets close to mean motion resonances. The observed misalignment likely reflects a primordial tilt of the stellar spin axis relative to the protoplanetary disk, rather than dynamical interactions. HIP 41378 f is the first non-eccentric long-period planet (P>100 days) observed with the RM effect, opening new constraints on planetary formation theories. This observation should motivate the exploration of planetary obliquities across a longer range of orbital distances through international collaboration.truePlanets and satellites: individual: HIP 41378 | Stars: rotation | Techniques: spectroscopicArticle143207461 September 20250A173arArticle