Comparison of the observed wavelength-dependent displacement δ (filled circles) and best model predictions (thick black lines) for the first CO bandhead. Each panel refers to a different projected baseline, as labelled. At the distance of 51 Oph (123 pc) 0.8 mas corresponds to 0.1 au. The shifts are calculated for a ring of radius 0.1 au, inclination of 70°, and v_rot = 140 km s⁻¹. The vertical dashed line shows the peak of the CO bandhead. The physical parameters of the model are T = 2400 K, N_CO = 4 × 10²¹ cm⁻², Δv = 15 km s⁻¹, and v_rot sini = 130 km s⁻¹.