Circumstellar molecular maser emission of AGB and post-AGB stars

Results of long-term studies of circumstellar molecular maser emission of late-type giant and supergiant variable stars are reported. In the 1.35-cm H₂O line, the peak flux density correlates with the optical brightness lagging behind it by 0.3-0.4 P (P is the stellar period). "Superperiods" of 10 to 15 P are visible in several stars, demonstrated as high maxima in the visible light curve and associated flares in the H₂O maser line. In the 18-cm OH lines, full polarization of the maser emission has been measured. Variable Zeeman patterns suggesting a changing magnetic field of a few milligauss have been detected.