The ¹²CH₃OH and ¹³CH₃OH spectra towards W51 e1/e2. For both isotopologues the velocity has been set to zero at the frequency of the lowest energy line (see Table 1), and the upper and lower sets of vertical lines mark the expected line velocities for ¹²CH₃OH and ¹³CH₃OH, respectively. The dashed box marks the velocity interval over which the intensities, presented in Table 3, have been integrated. Note that the intensity of the ¹³CH₃OH spectrum has been scaled up by a factor of 10.