Context. Cool, evolved stars undergo copious mass loss but the detailed mechanisms and the form in which the matter is returned to the ISM are still under debate. Aims. We investigated the structure and evolution of the wind at 5 to 50 stellar radii from asymptotic giant branch and red supergiant stars. Methods. 22-GHz water masers around seven evolved stars were imaged using MERLIN, at sub-AU resolution. Each source was observed at between 2 and 7 epochs, covering several stellar periods. We compared our results with long-term single dish monitoring provided by the Pushchino radio telescope. Results. The 22-GHz emission is located in approximately spherical, thick, unevenly filled shells. The outflow velocity increases twofold or more between the inner and outer shell limits. Water maser clumps could be matched at successive epochs separated by less than two years for AGB stars, or at least 5 years for RSG. This is much shorter than the decades taken for the wind to cross the maser shell, and comparison with spectral monitoring shows that some features fade and reappear. In five sources, most of the matched maser features brighten or dim in concert from one epoch to the next. A number of individual maser features show idiosyncratic behaviour, including one cloud in W Hya caught in the act of passing in front of a background cloud leading to 50-fold, transient amplification. The masing clouds are one or two orders of magnitude denser than the wind average and contain a substantial fraction of the mass loss in this region, with a filling factor <1%. The RSG clouds are about ten times bigger than those round the AGB stars. Conclusions. Proper motions are dominated by expansion, with no systematic rotation. The maser clouds presumably survive for decades (the shell crossing time) but the masers are not always beamed in our direction. Only radiative effects can explain changes in flux density throughout the maser shells on short timescales. The size of the clouds is proportional to that of the parent star, being of a similar radius to the star once the clumps reach the 22-GHz maser shell. Stellar properties such as convection cells must determine the clumping scale.

Abstract. We present the results from observing the circumstellar maser emission of the M-type supergiant S Per in the 6 16 -5 23 water-vapour line at 1.35 cm. The observations were carried out in 1981-2002 (JD = 2 444 900-2 452 480) on the RT-22 radio telescope of the Pushchino Radio Astronomy Observatory, Astrospace Center of the Lebedev Institute of Physics, Russian Academy of Sciences. The H 2 O spectra obtained represent an unprecedented long, uniform dataset on this star. We discuss the properties of the optical and maser variations of S Per, together with particulars of the available VLBI maps. The close relation between maser and optical variations favors a model in which mass-loss is episodic. Changes observed in the total H 2 O line flux follow the visual light curve with a delay of 0.01 to 0.5P, where P ≈ 800 d is the mean light cycle for S Per. The feature at V LSR = −44 km s −1 flared in July 1988, which seemed to be the response of the maser to an unusually bright optical maximum. The position of the −44-km s −1 feature on the VLBI maps coincides with the direction toward the optical stellar disc, which can be explained by amplification of enhanced stellar continuum by the H 2 O line.

Abstract. H 2 O line observations at λ = 1.35 cm of the Mira Ceti-type variable star U Ori are reported. The observations cover the time interval from March, 1980, to September, 1999. Variations of the integral flux and velocity centroid of the H 2 O line are analysed. The flux in general correlates with the visual light curve, following it with some phase delay ∆ϕ ∼ 0.2 − 0.4P (P is the period of the star). The maser emission is generated in a quasistationary layer of gas and dust at a distance of about 10 14 cm from the stellar centre. The maser variability is explained by the action of periodic shocks, driven by stellar pulsation and arriving to the maser in each stellar cycle. The shocks provide the maser pumping, whereas the sink of the waste energy is controlled by the dust, periodically heated by the stellar radiation near the light maximum; this accounts for the correlation of the maser radiation maximum with the descending branch of the light curve. Temporary weakness of the maser emission may be due to decay of the quasi-stationary layer, which is then rebuilt by a powerful shock, carrying away from the star a portion of the lost mass, once per a few stellar periods -the "superperiod". In its turn, the superperiod may reflect multiperiodic pulsation of the star or the presence of a long-term activity cycle, connected with restructuring of the stellar magnetic field, which is known to be strong in U Ori.

Abstract.The results of observations of the H2O maser emission (λ = 1.35 cm) of the Mira-type variable star RS Vir are presented. The observations were carried out in 1981-2001 (JD = 2 444 900−2 451 995) on the RT-22 radio telescope of the Pushchino Radio Astronomy Observatory (Russia). The variability curve of the H2O maser emission integrated flux Fint(H2O) of RS Vir correlates with its visual light curve with some phase delay ∆ϕ = (0.05−0.35)P (P is the star's period). The delay variations seem to be periodic at a timescale of 19-20 years ("superperiod" of the maser variations). If the variability of the H2O maser RS Vir is caused by periodic action of pulsation-driven shock waves, the shock travel time from the photosphere to the inner boundary of the H2O maser shell can be as long as (3-5)P . Another explanation is that maser variations may be connected with changing gas density in the masering region of the circumstellar envelope. In particular, long-term maser variations (the "superperiod") may be caused by changing mass loss rateṀ .

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