Continuum subtracted spectrum of 51 Oph around the CO bandhead emission for the May dataset (black dots) normalised to the peak of the first bandhead. Overplotted is the model spectrum (red solid line) computed for T = 2400 K, N_CO = 4 × 10²¹ cm⁻², Δv = 15 km s⁻¹ and v_rot sin i = 130 km s⁻¹. The verticalblack line at the upper left of the first bandhead represent twice the rms value measured in the observed continuum region adjacent to the first bandhead.