The intensities of the different peaks are proportional to the abundances of the different isotopologues. From simple statistical arguments, assuming a random distribution of O nuclei amongst the oxygen atoms, the abundances of the species giving rise to the three peaks are proportional to {(1 -x) 2 , 2x(1 -x), x 2 }, where x is the abundance of 18 O nuclei, and 1 -x is the abundance of O nuclei. The observed spectral amplitudes are consistent with an 18 O enrichment level of x ≃ 46%. This corresponds well to the known composition of the solvent (see section II A).