N(CO)/N(CO₂) as a function of the position of the centre of the 2.70 µm CO₂ band for individual double-dip objects (orange crosses) and cliff objects (red squares). The shadowed regions in the graph represent the range of points we obtained during CO₂ irradiation experiments at 45 K (orange) and CH₃OH irradiation experiments, pure and mixed with H₂O, at 45–60 K (light red). The horizontal dashed red line represents a lower limit for cliff object 1999 OX₃ , which lacks the 2.7 µm CO₂ band. Huya (grey cross) is clustered as a double-dip, but behaves like a cliff in terms of CO and CO₂. Conversely, 2004 PG₁₁₅ (grey square) is clustered as a cliff, but behaves like a double-dip in terms of CO and CO₂.