Zoom-in of the 13.5–17.5 µm region of the CX Tau spectrum (black), together with all slab model fits to the molecular features. These models include C₂H₂ (yellow), HCN (orange), ¹²CO₂ (green), ¹³CO₂ (purple), H₂O (blue), and OH (magenta). In light pink, we show a slab model demonstrating a potential detection of CO ¹⁸O. The bottom panel shows the data in black, the combined model with and without the CO ¹⁸O model in light pink and red, respectively. The red bar indicates the window in which the uncertainty was estimated. An artifact around 16.15 µm has been masked. The insets show close-ups of the regions around the ¹³CO₂ emission feature and the (potential) CO¹⁸O feature.