Observed spectra (in blue) around the ¹⁶O¹⁸O transition at 233.946098 GHz towards five positions in IRAS 16293 B depicted in the maps in Fig. 2. Red and green lines show the best-fit curves to the data assuming a Gaussian profile and the profile from the CH₃ NCO transition at 234.08809 GHz, respectively. Residual spectra of the best-fit Gaussian and reference transition profiles are shown below the observed spectra in red and green, respectively. Positions in arcseconds relative to the continuum peak position of IRAS 16293 B are shown at the top left of each panel. The vertical and horizontal black dashed lines depict the source velocity at 2.7 km s⁻¹ and the baseline, respectively.