HD 89345: a bright oscillating star hosting a transiting warm Saturn-sized planet observed by K2

We report the discovery and characterization of HD 89345b (K2-234b; EPIC 248777106b), a Saturn-sized planet orbiting a slightly evolved star. HD 89345 is a bright star (V = 9.3 mag) observed by the K2 mission with 1 min time sampling. It exhibits solar-like oscillations. We conducted asteroseismology to determine the parameters of the star, finding themass and radius to be 1.12(-0.01)(+0.04) and 1.657(-0.004)(+0.020)R(circle dot), respectively. The star appears to have recently left the main sequence, based on the inferred age, 9.4(-1.3)(+0.4)Gyr, and the non-detection of mixed modes. The star hosts a 'warm Saturn' (P = 11.8 d, R-p = 6.86 +/- 0.14 R-circle plus). Radial-velocity follow-up observations performed with the FIbre-fed Echelle Spectrograph, HARPS, and HARPS-N spectrographs show that the planet has a mass of 35.7 +/- 3.3 M-circle plus. The data also show that the planet's orbit is eccentric (e approximate to 0.2). An investigation of the rotational splitting of the oscillation frequencies of the star yields no conclusive evidence on the stellar inclination angle. We further obtained Rossiter-McLaughlin observations, which result in a broad posterior of the stellar obliquity. The planet seems to confirm to the same patterns that have been observed for other sub-Saturns regarding planet mass and multiplicity, orbital eccentricity, and stellar metallicity.