ИСТИНА

We report long-term observations of H2O and OH maser emission sources at wavelengths of 1.35 and 18 cm associated with star-forming regions. The observations were carried out, respectively, on the 22-metre radio telescope of the Pushchino Radio Astronomy Observatory, Russia, and on the radio telescope of the Nançay Radio Astronomy Station, France. The sample includes about 150 sources. Some of them are well-known masers, such as Ori A, W43M3, W51, W75N. Strong quasi-periodic flares of maser emission have been observed. We invoke, where available, inteferometric data on the sources and trace the fate of individual maser features, the variations of their flux densities and radial velocities. In particular, the velocity drift of some features on a timescale of several years was observed. Possible causes of this are discussed, among them consecutive excitation of the maser by a propagating shock wave. Other sources are new, recently discovered, strong emitters in the H2O and OH lines asociated with quite young star-forming regions traced by Class I methanol masers radiating in the 44 GHz 70-61A+ transition of CH3OH. We attempt to range the sources observed according to the presence or absence of this or that maser (CH3OH, H2O, OH), thus creating an evolutionary sequence of the star-forming regions hosting these masers.