Posterior distribution of the GP fit to the RV data in the α_GP vs. P_rot plane for GJ 251. The color-coding shows the log-likelihood normalized to the highest value in the posterior sample. Gray samples indicate solutions with a Δ ln L > 10 compared to the best solution. Top: GP fit to the RV data with a wide uniform prior for the rotational period. Bottom: GP fit to the RV data with an informative normal prior based on the photometric GP results and additional constraints on the other hyperparameters. We overplot the derived rotational period of the photometric GP and its 3σ uncertainties with vertical lines. The horizontal line marks the cut in α_GP used to constrain the GP fit shown in the lower plot.