Co-propagation of maser emission at 1720 and 4765 MHz

MERLIN observations are presented of OH 4765-MHz and OH 1720-MHz masers in the massive star-forming region W3(OH). Two of the three intense spots of m aser emission at 4765 MHz are spatially coincident with two similar spots a t 1720 MHz in both left-hand circular (LHC) and right-hand circular (RHC) p olarizations, to an accuracy of 15 mas. The spots also overlap in velocity when allowance is made for Zeeman splitting of the 1720-MHz Line. We conclu de that we have found two examples of masers in different rotational levels of OH which are co-propagating through the same column of gas and experien cing competitive gain effects. The third 4765-MHz maser spot was found to h ave no overlapping counterpart amongst the 1720-MHz masers.