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We report precise radial velocity (RV) observations of HD 212657 (= K2-167),a star shown by K2 to host a transiting sub-Neptune-sized planet in a 10 dayorbit. Using Transiting Exoplanet Survey Satellite (TESS) photometry, werefined the planet parameters, especially the orbital period. We collected 74precise RVs with the HARPS-N spectrograph between August 2015 and October 2016.Although this planet was first found to transit in 2015 and validated in 2018,excess RV scatter originally limited mass measurements. Here, we measure a massby taking advantage of reductions in scatter from updates to the HARPS-N DataReduction System (2.3.5) and our new activity mitigation method called CCFActivity Linear Model (CALM), which uses activity-induced line shape changes inthe spectra without requiring timing information. Using the CALM framework, weperformed a joint fit with RVs and transits using EXOFASTv2 and find $M_p =6.3_{-1.4}^{+1.4}$ $M_{\oplus}$ and $R_p = 2.33^{+0.17}_{-0.15}$ $R_{\oplus}$,which places K2-167 b at the upper edge of the radius valley. We also findhints of a secondary companion at a $\sim$ 22 day period, but confirmationrequires additional RVs. Although characterizing lower-mass planets like K2-167b is often impeded by stellar variability, these systems especially help probethe formation physics (i.e. photoevaporation, core-powered mass loss) of theradius valley. In the future, CALM or similar techniques could be widelyapplied to FGK-type stars, help characterize a population of exoplanetssurrounding the radius valley, and further our understanding of theirformation.

Characterization of K2-167 b and CALM, a new stellar activity mitigation method