Constraint on the viewing angle from the absence of an observed source apparent displacement in our VLBA observations. In each panel, filled contours show the displacement of the centre of the fitted Gaussian expected between 44 d and 262 d, color coded as shown in the colorbar on the right, as a function of the E/A ratio and of the off-edge viewing angle θ_v − θ_j. The red contour shows Δ(262 d)−Δ(44 d) = 0.6 mas, which represents the largest displacement compatible at 1 σ with our observations. The red contour hence contains the excluded parameter region. The upper panel row refers to a wind-like external medium, while the lower row refers to a homogeneous external medium. Each column assumes a different jet half-opening angle, given at the top of the column.