DAMASSO, MarioMarioDAMASSOSCANDARIATO, GAETANOGAETANOSCANDARIATONASCIMBENI, VALERIOVALERIONASCIMBENINardiello, D.D.NardielloMancini, L.L.ManciniMarino, G.G.MarinoBRUNO, GiovanniGiovanniBRUNOBrandeker, A.A.BrandekerLETO, GiuseppeGiuseppeLETOMarzari, F.F.MarzariLANZA, Antonino FrancescoAntonino FrancescoLANZABENATTI, SerenaSerenaBENATTIDESIDERA, SilvanoSilvanoDESIDERABéjar, V. J. S.V. J. S.BéjarBiagini, A.A.BiaginiBORSATO, LUCALUCABORSATOCabona, L.L.CabonaCLAUDI, RiccardoRiccardoCLAUDILodieu, N.N.LodieuMAGGIO, AntonioAntonioMAGGIOMallorquín, M.M.MallorquínMESSINA, SergioSergioMESSINAMICELA, GiuseppinaGiuseppinaMICELARICCI, DavideDavideRICCISOZZETTI, AlessandroAlessandroSOZZETTISuárez Mascareño, A.A.Suárez MascareñoTURRINI, DiegoDiegoTURRINIZapatero Osorio, M. R.M. R.Zapatero Osorio2024-03-072024-03-0720230004-6361http://hdl.handle.net/20.500.12386/34909Context. The 20 Myr old star V1298 Tau hosts at least four planets. Since its discovery, this system has been a target of intensive photometric and spectroscopic monitoring. To date, the characterisation of its architecture and planets' fundamental properties has been very challenging. <BR /> Aims: The determination of the orbital ephemeris of the outermost planet V1298 Tau e remains an open question. Only two transits have been detected so far by Kepler/K2 and TESS, allowing for a grid of reference periods to be tested with new observations, without excluding the possibility of transit timing variations. Observing a third transit would allow for better constraints to be set on the orbital period and would also help in determining an accurate radius for V1298 Tau e because the previous transits showed different depths. <BR /> Methods: We observed V1298 Tau with the CHaracterising ExOPlanet Satellite (CHEOPS) to search for a third transit of planet e within observing windows selected to test three of the shortest predicted orbital periods. We also collected ground-based observations to verify the result found with CHEOPS. We reanalysed Kepler/K2 and TESS light curves to test how the results derived from these data are affected by alternative photometric extraction and detrending methods. <BR /> Results: We report the CHEOPS detection of a transit-like signal that could be attributed to V1298 Tau e. If so, that result would imply that the orbital period calculated from fitting a linear ephemeris to the three available transits is close to ~45 days. Results from the ground-based follow-up marginally support this possibility. We found that i) the transit observed by CHEOPS has a longer duration compared to that of the transits observed by Kepler/K2 and TESS; and ii) the transit observed by TESS is >30% deeper than that of Kepler/K2 and CHEOPS, and it is also deeper than the measurement previously reported in the literature, according to our reanalysis. <BR /> Conclusions: If the new transit detected by CHEOPS is found to be due to V1298 Tau e, this would imply that the planet experiences TTVs of a few hours, as deduced from three transits, as well as orbital precession, which would explain the longer duration of the transit compared to the Kepler/K2 and TESS signals. Another and a priori less likely possibility is that the newly detected transit belongs to a fifth planet with a longer orbital period than that of V1298 Tau e. Planning further photometric follow-up to search for additional transits is indeed necessary to solve the conundrum, as well as to pin down the radius of V1298 Tau e....STAMPAenArticle10.1051/0004-6361/2023468402-s2.0-85176803770https://www.aanda.org/articles/aa/full_html/2023/12/aa46840-23/aa46840-23.htmlhttps://api.elsevier.com/content/abstract/scopus_id/85176803770FIS/05 - ASTRONOMIA E ASTROFISICA