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Description
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 d on a moderately eccentric orbit (e=0.216_-0.026_^+0.027^). With a mass of M_*_=1.257_-0.029_^+0.029^ M_{sun}_, a radius of R_*_=2.341_-0.019_^+0.022^ R_{sun}_, 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.
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