Zoom-in of the CX Tau spectrum around the 15.07 µm feature which stands out in our residuals in Fig. A.1. Left: The CX Tau spectrum with the best-fit CO₂ model subtracted is shown in black, the best-fit H₂O model is shown in light blue, a 200 K, N = 10¹⁶ cm⁻² CO¹⁸O model shown in yellow and the combined H₂O and CO¹⁸O model is shown in red. The spectra of DR Tau and DF Tau (Temmink et al. 2024a,b; Grant et al. 2024), rescaled to the 15.17 µm H₂O feature in CX Tau, are shown in dark blue and purple, respectively. Right: The CX Tau spectrum with all best-fit models (¹²CO₂,¹³CO₂, H₂O, C₂H₂, HCN, and OH) subtracted is shown in black. Three CO¹⁸O models with N = 10¹⁶ cm⁻² and different temperatures are shown in yellow, orange, and brown.