ИСТИНА

We report long-term observations of H2O and OH maser emission sources at wavelengths of 1.35 and 18 cm associated with circumstellar envelopes of late-type variable stars. 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. A sample of ~70 late-type long-period variable stars has been monitored in the 1.35-cm H2O line in 1980-2015. It includes Mira-type stars (U Ori, RS Vir, U Her, R Cas,…) and semiregular variables (R Crt, RT Vir, W Hya, VX Sgr,…) We have traced H2O maser variations and found them to follow the optical brightness variations with a time delay of 0.3-0.4 stellar period. Some strong H2O flares were observed, which probably occur every 10 to 15 periods, probably due to long-term changes in the mass-loss rate; good examples are U Ori, U Her, and RS Vir. Especially spectacular flares were demonstrated by the semiregular star W Hya, when its H2O peak flux density reached several thousand janskys, while on the average it did not exceed 50-100 Jy. The model of shock excitation of the H2O maser in combination with stellar radiation pumping is considered. Results of long-term monitoring of time variations of circumstellar water maser sources in the semiregular M-supergiants S Per, VY CMa, VX Sgr, NML Cyg and non-variable M-supergiant IRC-10414 are reviewed. In addition to our single-dish data, very-long-baseline interferometry results are invoked. VX Sgr and IRC-10414 display a characteristic water-vapour line profile, which, together with VLBI data, suggests the presence of a rotating circumstellar disc and a bipolar outflow. In 2007-2015 70 long-period variable stars were observed (including miras and semiregular variables, M-type giants and supergiants) in the 18-cm OH lines. For 53 of them, the emission was detected in at least one of three OH lines (1612, 1665, or 1667 MHz). Circular and linear polarization of the maser emission was measured, yielding all four Stokes parameters. Polarized emission features were detected in the OH line spectra of 41 stars. A summary of all the observations is given. The results obtained for T Lep, R LMi, HU Pup, and R Crt are discussed in detail. Emission in the 1665 and 1667 MHz OH lines was detected in T Lep for the first time. Features probably due to Zeeman splitting were detected in the OH line profiles of all four stars. Estimates of the magnetic-field strengths in the maser sources were obtained (0.46-2.32 mG). Variability of the polarization characteristics of the maser emission of the stars on time intervals of several months was found.