Astrometry of star A relatively to star B. Left: all data available from the WDS (black) and results from this work (red), on Δα/ Δδ plane, reconstructed from ρ and θ. The star B is set in (0, 0) and marked with an asterisk. One can see a gradual movement with an average rate of 1.9(2) mas/yr and 3.5(3) mas/yr in α and δ respectively. Middle: enlargement of our results, showing Keck II/NIRC2 observations (both archival and ours). Each measurement (date, filter) is denoted with a different colour. The grey arrow and crosses show the vector of the gradual orbital movement since 2013-06-15 and predicted relative positions in dates of other observations (with uncertainties). Right: same measurements, but corrected for the gradual motion, and shifted, so the first Keck point is in (0, 0). Labels φ_S0 show phases of the 94.2-day orbit, according to the ephemeris used to clean the S0 curve from eclipses (Sect. 2.2). The data are clearly inconsistent with a P = 94.2 d, â ≃ 1.55 mas, i ≃ 90° orbit.