HD207897 b: A dense sub-Neptune transiting a nearby and bright K-type star

We present the discovery and characterization of a transiting sub-Neptune orbiting with a 16.20-day period around a nearby (28 pc) and bright (V= 8.37) K0V star HD207897 (TOI-1611). This discovery is based on photometric measurements from the Transiting Exoplanet Survey Satellite (TESS) mission and radial velocity (RV) observations from the SOPHIE, Automated Planet Finder (APF) and HIRES high precision spectrographs. We used EXOFASTv2 for simultaneously modeling the parameters of the planet and its host star, combining photometry and RVs data to determine the planetary system parameters. We show that the planet has a radius of $2.50\pm0.08$ R$_{\mathrm{E}}$ and a mass of either $14.4\pm 1.6$ M$_{\mathrm{E}}$ or $15.9\pm1.6$ M$_{\mathrm{E}}$ with nearly equal probability; the two solutions correspond to two possibilities for the stellar activity period. Hence, the density is either $ 5.1\pm0.7$ g cm$^{-3}$ or $5.5^{+0.8}_{-0.7}$ g cm$^{-3}$, making it one of the relatively rare dense sub-Neptunes. The existence of such a dense planet at only 0.12 AU from its host star is unusual in the currently observed sub-Neptune ($2 < R_{\mathrm{E}} < 4$) population. The most likely scenario is that this planet has migrated to its current position.