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Published online by Cambridge University Press: 07 August 2017
We present the first VLBI measurement of interstellar ammonia (NH3) masers. Two masers were found toward the ultracompact Hii regions, W51-e1 and e2. The masers are unresolved in angle and smaller than 0.1 milliarcseconds. Unless these masers are highly beamed, they appear to be saturated.