We report the discovery of a warm sub-Saturn , TOI-257b ( HD 19916b ) , based on data from NASA ’ s Transiting Exoplanet Survey Satellite ( TESS ) .
The transit signal was detected by TESS and confirmed to be of planetary origin based on radial-velocity observations with the Minerva -Australis telescope array .
An analysis of the TESS photometry , the Minerva -Australis , FEROS , and HARPS radial velocities , and the asteroseismic data of the stellar oscillations reveals that TOI-257b has a mass of M _ { P } = 0.134 ^ { +0.023 } _ { -0.022 } { M _ { J } } ( 42.6 ^ { +7.3 } _ { -7.0 } M _ { \oplus } ) , a radius of R _ { P } = 0.626 ^ { +0.013 } _ { -0.012 } { R _ { J } } ( 7.02 ^ { +0.15 } _ { -0.13 } R _ { \oplus } ) , and an orbit with eccentricity 0.242 ^ { +0.040 } _ { -0.065 } and period 18.38827 \pm 0.00072 days .
TOI-257b orbits a bright ( \mathrm { V } = 7.570 mag ) somewhat evolved late F-type star with M _ { * } = 1.390 \pm 0.046 { M _ { sun } } , R _ { * } = 1.888 \pm 0.033 { R _ { sun } } , T _ { eff } = 6075 \pm 90 K , and v \sin { i } = 11.3 \pm 0.5 km s ^ { -1 } .
Additionally , we statistically validate a second non-transiting sub-Saturn mass planet on a \sim 71 day orbit using the radial velocity data .
This system joins the ranks of a small number of exoplanet host stars that have been characterized with asteroseismology .
Warm sub-Saturns are rare in the known sample of exoplanets , and thus the discovery of TOI-257b is important in the context of future work studying the formation and migration history of similar planetary systems .