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We report the discovery and characterization of HD 89345b (K2-234b; EPIC248777106b), a Saturn-sized planet orbiting a slightly evolved star. HD 89345is a bright star ($V = 9.3$ mag) observed by the K2 mission with one-minutetime sampling. It exhibits solar-like oscillations. We conductedasteroseismology to determine the parameters of the star, finding the mass andradius to be $1.12^{+0.04}_{-0.01}~M_\odot$ and$1.657^{+0.020}_{-0.004}~R_\odot$, respectively. The star appears to haverecently left the main sequence, based on the inferred age,$9.4^{+0.4}_{-1.3}~\mathrm{Gyr}$, and the non-detection of mixed modes. Thestar hosts a "warm Saturn" ($P = 11.8$~days, $R_p = 6.86 \pm 0.14~R_\oplus$).Radial-velocity follow-up observations performed with the FIES, HARPS, andHARPS-N spectrographs show that the planet has a mass of $35.7 \pm3.3~M_\oplus$. The data also show that the planet's orbit is eccentric($e\approx 0.2$). An investigation of the rotational splitting of theoscillation frequencies of the star yields no conclusive evidence on thestellar inclination angle. We further obtained Rossiter-McLaughlinobservations, which result in a broad posterior of the stellar obliquity. Theplanet seems to conform to the same patterns that have been observed for othersub-Saturns regarding planet mass and multiplicity, orbital eccentricity, andstellar metallicity.

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