TOI-1296b and TOI-1298b observed with TESS and SOPHIE: Two hot Saturn-mass exoplanets with different densities around metal-rich stars

We present the discovery of two new transiting extrasolar planet candidates identified as TOI-1296.01 and TOI-1298.01 by the Transiting Exoplanet Survey Satellite (TESS). The planetary nature of these candidates has been secured with the SOPHIE high-precision spectrograph through the measurement of the companion’s mass with the radial velocity method. Both planets are similar to Saturn in mass and have similar orbital periods of a few days. They, however, show discrepant radii and therefore different densities. The radius discrepancy might be explained by the different levels of irradiation by the host stars. The subgiant star TOI-1296 hosts a low-density planet with 1.2 RJup while the less luminous, lower-size star TOI-1298 hosts a much denser planet with a 0.84 RJup radius, resulting in bulk densities of 0.198 and 0.743 g.cm−3, respectively. In addition, both stars are strongly enriched in heavy elements, having metallicities of +0.44 and +0.49 dex, respectively. The planet masses and orbital periods are 0.298± 0.039 MJup and 3.9443715± 5.8 10−6 days for TOI-1296b, and 0.356± 0.032 MJup and 4.537164± 1.2 10−5 days for TOI-1298b. The mass measurements have a relative precision of better than 13%.