Continuum-subtracted spectrum of 51 Oph normalised to the peak intensity of the first bandhead (black dots). Predictions of theoretical CO ring models as described in the main text are overplotted (solid coloured lines). The vertical black line in the upper left corner of the first bandhead represent twice the rms value measured in the observed continuum region adjacent to the first bandhead. The models have been computed for combinations of T and N_CO values of T = 1800 K, N_CO = 1 × 10²² cm⁻² (green), T = 2400 K, N_CO = 4 × 10²¹ cm⁻² (red), and T = 2900 K, N_CO = 8 × 10²⁰ cm⁻² (blue) with aconstant value of Δv = 15 km s⁻¹, and v_rot sini = 130 km s⁻¹.