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Published online by Cambridge University Press: 14 August 2015
We analyze in detail the pumping of 18 cm OH masers by the overlap of far infra-red lines in HII/OH regions and in circumstellar shells. We present some results of a model of pumping of HII/OH masers by thermal overlap. Pumping by overlap appears to be quite general but very complicated. It can account for the different observed main-line masers. However, other types of pumping, mainly by collisions with H or by near infra-red radiation, are possible if certain conditions are met for the physical and astrophysical parameters.