TOI-5108 b and TOI 5786 b: Two transiting sub-Saturns detected and characterized with TESS, MaHPS, and SOPHIE

We report the discovery and characterization of two sub-Saturns from the Transiting Exoplanet Survey Satellite ( TESS ) using high-resolution spectroscopic observations from the MaHPS spectrograph at the Wendelstein Observatory and the SOPHIE spectrograph at the Haute-Provence Observatory. Combining photometry from TESS, KeplerCam, LCOGT, and MuSCAT2, along with the radial velocity measurements from MaHPS and SOPHIE, we measured precise radii and masses for both planets. TOI-5108,b is a sub-Saturn, with a radius of $6.6 ± 0.1$ R_⊕ and a mass of $32 ± 5$ M_⊕. TOI-5786,b is similar to Saturn, with a radius of $8.54 ± 0.13$ R_⊕ and a mass of $73 ± 9$ M_⊕. The host star for TOI-5108 b is a moderately bright (Vmag 9.75) G-type star. TOI-5786 is a slightly dimmer (Vmag 10.2) F-type star. Both planets are close to their host stars, with periods of 6.75 days and 12.78 days, respectively. This puts TOI-5108,b just within the bounds of the Neptune desert, while TOI-5786,b is right above the upper edge. We estimated hydrogen-helium (H/He) envelope mass fractions of $38,%$ for TOI-5108,b and $74,% $ for TOI-5786,b. However, when using a model for the interior structure that includes tidal effects, the envelope fraction of TOI-5108,b could be much lower (∼ 20,%), depending on the obliquity. We estimated mass-loss rates between 1.0 times 10^9,g/s and 9.8 times 10^9,g/s for TOI-5108,b and between $3.6 10^8$,g/s and $3.5 10^9$,g/s for TOI-5786,b. Given their masses, both planets could be stable against photoevaporation. Furthermore, at these mass-loss rates, there is likely no detectable signal in the metastable helium triplet with the James Webb Space Telescope (JWST). We also detected a transit signal for a second planet candidate in the TESS data of TOI-5786, with a period of 6.998 days and a radius of $3.83 ± 0.16$ R_⊕. Using our RV data and photodynamical modeling, we were able to provide a 3-σ upper limit of 26.5 M_⊕ for the mass of the potential inner companion to TOI-5786,b.