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... Vologaeses, cognitis Corbulonis rebus regemque alienigenam Tigranen Armeniae impositum,...

regemque...impositum: o.o, depending on "cognitis," which is the participle in the prior abl. abs. clause: Vologes was aware of the activities of the Roman general Corbulo and that a foreigner Tigranes had been place <on the throne> of Armenia.