2004MNRAS.350.1409N

We present the first maps of 4765-MHz OH masers in two star-forming regions Cepheus A and W75N, made with MERLIN. In Cep A, the emission has an arc-like structure of size 40 mas with a clear velocity gradient, and lies at the edge of HII region 3b. Over a period of 8 weeks, the maser diminished in intensity by a factor of 7 but its structure remained stable. This structure coincides with a newly mapped 1720-MHz maser in Cep A within the positional error, and matches it in velocity. No emission of any other ground-state or 4.7-GHz excited OH transitions was detected in the 4765/1720-MHz region. The 1720-MHz line exhibits Zeeman splitting that corresponds to a magnetic field strength of -17.3 mG. In W75N, the excited 4765-MHz OH maser has a linear structure of size 45 mas with a well-defined velocity gradient, and lies at the edge of HII region VLA 1. This structure coincides in position and velocity with the 1720-MHz masers. We conclude that in both sources the 4765-MHz emission coexists with 1720-MHz emission in the same volume of gas. In such a case, the physical conditions in these regions are tightly constrained by the maser-pumping models.