GRAVITY observations for the central spectral channel of the Fringe Tracker (λ = 2.15 μm). Left. Visibilities squared. Middle. Closure phases. The blue symbols at the bottom of the visibility and closure phase curves display the absolute values of differences between the observations and the best-fit ring models without azimuthal modulation. These residuals are shifted by -5° for the closure phase plots. Right. (u, v) planes: (u, v) coordinates in blue, (-u, -v) coordinates in red, and 10, 20, 30, 40, 50, and 60 Mλ isocontours in dotted lines.