Southworth, JohnJohnSouthworthDominik, M.M.DominikJorgensen, U. G.U. G.JorgensenAndersen, M. I.M. I.AndersenBozza, V.V.BozzaBurgdorf, M. J.M. J.BurgdorfD'Ago, G.G.D'AgoDib, S.S.DibFiguera Jaimes, R.R.Figuera JaimesFujii, Y. I.Y. I.FujiiGill, S.S.GillHaikala, L. K.L. K.HaikalaHinse, T. C.T. C.HinseHundertmark, M.M.HundertmarkKhalouei, E.E.KhaloueiKorhonen, H.H.KorhonenLonga-Pena, P.P.Longa-PenaMancini, L.L.ManciniPeixinho, N.N.PeixinhoRabus, M.M.RabusRahvar, S.S.RahvarSajadian, S.S.SajadianSkottfelt, J.J.SkottfeltSnodgrass, C.C.SnodgrassSpyratos, P.P.SpyratosTregloan-Reed, J.J.Tregloan-ReedUnda-Sanzana, E.E.Unda-Sanzanavon Essen, C.C.von Essen2025-12-302025-12-3020191365-2966https://hdl.handle.net/20.500.12740/24150Transits in the planetary system WASP-4 were recently found to occur 80 s earlier than expected in observations from the TESS satellite. We present 22 new times of mid-transit that confirm the existence of transit timing variations, and are well fitted by a quadratic ephemeris with period decay dP/dt=-9.2 +/- 1.1 ms yr(-1). We rule out instrumental issues, stellar activity, and the Applegate mechanism as possible causes. The light-time effect is also not favoured due to the non-detection of changes in the systemic velocity. Orbital decay and apsidal precession are plausible but unproven. WASP-4 b is only the third hot Jupiter known to show transit timing variations to high confidence. We discuss a variety of observations of this and other planetary systems that would be useful in improving our understanding of WASP-4 in particular and orbital decay in general.Acceso Abiertostars: activitystars: fundamental parametersstars: individual: WASP-4http://dx.doi.org/10.1093/mnras/stz2602