HD 1397b: A Transiting Warm Giant Planet Orbiting A V=7.8 mag Subgiant Star Discovered by TESS

We report the discovery of a transiting planet first identified as a candidate in Sector 1 of the Transiting Exoplanet Survey Satellite (TESS), and then confirmed with precision radial velocities. HD 1397b has a mass of M-P = 0.367(-0.023)(+0.022)M(J), a radius of R-P = 1.023-(+0.013)(0.013) R-J, and orbits its bright host star (V= 7.8 mag) with an orbital period of 11.5366 +/- 0.0003 don a moderately eccentric orbit (e = 0 .216(-0.026)(+0.027)) With a mass of M-* = 1.257(-0.029)(+0.029) M-circle dot, a radius of R-* = 2.341(-0.019)(+0.022 )R(circle dot), and an age of 4.46 +/- 0.25 Gyr, the solar-metallicity host star has already departed from the main sequence. We find evidence in the radial velocity measurements of a secondary signal with a longer period. We attribute it to the rotational modulation of stellar activity, but a long-term radial velocity monitoring would be necessary to discard if this signal is produced by a second planet in the system. The HD 1397 system is among the brightest ones currently known to host a transiting planet, which will make it possible to perform detailed follow-up observations in order to characterize the properties of giant planets orbiting evolved stars.