Projected separation of β Pictoris b as a function of time. The one-planet and two-planet model fits that use only the b astrometry are shown in red and cyan, respectively. These models are described in Sect. 4. The two-planet model fit that uses b and c astrometry is also plotted, in blue, and the fit that also uses the RV is plotted in purple (Sect. 5). Top row: orbit models and all of the data. Middle row: residuals after subtracting a pure Keplerian orbit for planet b based on the orbital parameters from the two-planet model using b and c astrometry. Bottom row: residual after accounting for the perturbation of planet c. We note that, although the red one-planet model is a pure Keplerian orbit, it is not a flat line in the middle row because the best-fit one-planet Keplerian model also attempts to fit the perturbations due to the second planet.