The relation between the 12C/13C isotopic ratio in C18O and CH3OH for the observed sources (stars), with the dashed line representing a 1:1 relation plotted to facilitate interpretation. Ratios and errors are those found in Table 3, the source with the relatively low methanol ratio is W51 e1/e2. The black triangle marks the 12C/13C ratio in methanol versus solid C16O towards NGC 7538 IRS9, see Sect. 6.
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