Mid-transit (T2-T3) transmission spectrum around the HeI triplet for nights 1, 2 and 3, from top to bottom, respectively. The black line shows the spectral regions unaffected by OH⁻ lines, while the red line marks the spectral regions affected, and corrected for, OH⁻ emission. The blue vertical lines mark the helium triplet line center positions. Note the different absorption scale between night 1 and nights 2 and 3.