2001MNRAS.324..643G

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 maser emission at 4765MHz are spatially coincident with two similar spots at 1720MHz in both left-hand circular(ref???[q1]) (LHC) and right-hand circular(ref???[q1]) (RHC) polarizations, to an accuracy of 15mas. The spots also overlap in velocity when allowance is made for Zeeman splitting of the 1720-MHz line. We conclude 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 experiencing competitive gain effects. The third 4765-MHz maser spot was found to have no overlapping counterpart amongst the 1720-MHz masers.