TOI-1199 b and TOI-1273 b: Two new transiting hot Saturns detected and characterized with SOPHIE and TESS

We report the characterization of two planet candidates detected by the Transiting Exoplanet Survey Satellite (TESS), TOI-1199 b and TOI-1273 b, with periods of 3.7 and 4.6 days, respectively. Follow-up observations for both targets, which include several ground-based light curves, confirmed the transit events. High-precision radial velocities from the SOPHIE spectrograph revealed signals at the expected frequencies and phases of the transiting candidates and allowed mass determinations with a precision of $8.4\mathrm{\%}$ and $6.7\mathrm{\%}$ for TOI-1199 b and TOI-1273 b, respectively. The planetary and orbital parameters were derived from a joint analysis of the radial velocities and photometric data. We find that the planets have masses of $0.239\pm 0.020M_{\mathrm{J}}$ and $0.222\pm 0.015M_{\mathrm{J}}$ and radii of $0.938\pm 0.025R_{\mathrm{J}}$ and $0.99\pm 0.22R_{\mathrm{J}}$, respectively. The grazing transit of TOI-1273 b translates to a larger uncertainty in its radius, and hence also in its bulk density, compared to TOI-1199 b. The inferred bulk densities of $0.358\pm 0.041\mathrm{g}{\mathrm{c}\mathrm{m}}^{-3}$ and $0.28\pm 0.11\mathrm{g}{\mathrm{c}\mathrm{m}}^{-3}$ are among the lowest known for exoplanets in this mass range, which, considering the brightness of the host stars ($V\approx 11\mathrm{m}\mathrm{a}\mathrm{g}$), render them particularly amenable to atmospheric characterization via the transit spectroscopy technique. The better constraints on the parameters of TOI-1199 b provide a transmission spectroscopy metric of $134\pm 17$, making it the better suited of the two planets for atmospheric studies.