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We identify a sub-Neptune exoplanet ($R_p = 2.5 \pm 0.2$ R$_\oplus$)transiting a solar twin in the Ruprecht 147 star cluster (3 Gyr, 300 pc, [Fe/H]= +0.1 dex). The ~81 day light curve for EPIC 219800881 (V = 12.71) from K2Campaign 7 shows six transits with a period of 13.84 days, a depth of ~0.06%,and a duration of ~4 hours. Based on our analysis of high-resolution MIKEspectra, broadband optical and NIR photometry, the cluster parallax andinterstellar reddening, and isochrone models from PARSEC, Dartmouth, and MIST,we estimate the following properties for the host star: $M_\star = 1.01 \pm0.03$ M$_\odot$, $R_\star= 0.95 \pm 0.03$ R$_\odot$, and $T_{\rm eff} = 5695\pm 50$ K. This star appears to be single, based on our modeling of thephotometry, the low radial velocity variability measured over nearly ten years,and Keck/NIRC2 adaptive optics imaging and aperture-masking interferometry.Applying a probabilistic mass-radius relation, we estimate that the mass ofthis planet is $M_p = 7 +5 -3$ M$_\oplus$, which would cause a RVsemi-amplitude of $K = 2 \pm 1$ m s$^{-1}$ that may be measurable with existingprecise RV facilities. After statistically validating this planet with BLENDER,we now designate it K2-231 b, making it the second sub-stellar object to bediscovered in Ruprecht 147 and the first planet; it joins the small but growingranks of 23 other planets found in open clusters.

K2-231 b: A sub-Neptune exoplanet transiting a solar twin in Ruprecht 147