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Published online by Cambridge University Press: 25 April 2016
In the terrestrial environment the abundance ratio of 18O to the common oxygen isotope 16O is about 1 in 490. Measurements of hydroxyl absorption in the interstellar medium show that 18OH as well as 16OH can be detected towards Sgr A and Sgr B2 (Gardner et al. 1970; Wilson and Barrett 1970; Williams and Gardner 1981); in these two galactic centre regions the 18OH isotopic species has an abundance relative to 16OH of about 1 in 220, somewhat greater than the terrestrial ratio of 18O to 16O.