Spectacular flares are reported in the excited OH 4765-MHz maser emission from Mon R2. The maser has increased in flux by over two orders of magnitude, with a shortest time-scale of only 19 d to double in intensity. The linewidth and radial velocity of the emission peak have also varied systematically during the flaring episodes. MERLIN measurements show that the 4765-MHz emission comes from two spots associated with the infrared source Mon R2-IRS3. The components are separated by 0.17 arcsec, which corresponds to a projected distance of only 140 au. The emission from each spot is linearly...

Spectacular flares are reported in the excited OH 4765-MHz maser emission from Mon R2. The maser has increased in flux by over two orders of magnitude, with a shortest time-scale of only 19 d to double in intensity. The linewidth and radial velocity of the emission peak have also varied systematically during the flaring episodes. MERLIN measurements show that the 4765-MHz emission comes from two spots associated with the infrared source Mon R2-IRS3. The components are separated by 0.17 arcsec, which corresponds to a projected distance of only 140 au. The emission from each spot is linearly polarized, with identical position angles which are approximately orthogonal to the direction of the molecular outflow from Mon R2.