A Transiting Giant on a 7.7 yr Orbit Revealed by Transit Timing Variations in the TOI-201 System

We report the detection and characterization of TOI-201 c, a long-period transiting companion to the warm Jupiter TOI-201 b. Its presence was first inferred from high-amplitude transit timing variations (TTVs) in TOI-201 b, pointing to a massive outer body on a 7 . 7 − 0.6 + 1.0 yr eccentric orbit. This prediction was confirmed when Transiting Exoplanet Survey Satellite observed a transit of TOI-201 c, precisely constraining its orbital geometry. A joint fit to TTVs, transit photometry, and archival radial velocities yields a mass of 14 . 2 − 1.2 + 1.0 M Jup and an eccentricity of 0.64 3 − 0.021 + 0.009 . The mutual inclination between planets b and c is 2 . ° 9 − 4.4 + 4.8 , indicating a nearly coplanar architecture. Long-term numerical integrations confirm dynamical stability over gigayear timescales and predict that transits of TOI-201 b will cease within a few thousand years. TOI-201 c ranks among the longest-period transiting planets with well-constrained properties. Its detection via TTVs, followed by a confirmed transit, represents a rare observational sequence and highlights the power of TTVs and photometric monitoring to uncover distant companions. The TOI-201 system offers a valuable laboratory for testing models of giant planet formation, migration, and secular evolution in multiplanet systems.