TOI-199 b: A Well-characterized 100 day Transiting Warm Giant Planet with TTVs Seen from Antarctica

We present the spectroscopic confirmation and precise mass measurement of the warm giant planet TOI-199 b. This planet was first identified in TESS photometry and confirmed using ground-based photometry from ASTEP in Antarctica including a full 6.5 h long transit, PEST, Hazelwood, and LCO; space photometry from NEOSSat; and radial velocities (RVs) from FEROS, HARPS, CORALIE, and CHIRON. Orbiting a late G-type star, TOI-199 b has a \(104.854^{+0.001}_{-0.002}\) d period, a mass of 0.17 ± 0.02 M\(_\mathrm{J}\), and a radius of 0.810 ± 0.005 R\(_\mathrm{J}\). It is the first warm exo-Saturn with a precisely determined mass and radius. The TESS and ASTEP transits show strong transit timing variations, pointing to the existence of a second planet in the system. The joint analysis of the RVs and TTVs provides a unique solution for the non-transiting companion TOI-199 c, which has a period of \(273.69^{+0.26}_{-0.22}\) d and an estimated mass of \(0.28^{+0.02}_{-0.01}\) M\(_\mathrm{J}\). This period places it within the conservative Habitable Zone.

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