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We wish to understand the reasons for the strong variability of H2O masers incircumstellar shells of late-type stars. We monitored RX Boo and SV Peg in the22-GHz maser line of water vapour with single-dish telescopes. The monitoringperiod covered two decades for RX Boo (1987 -- 2007) and 12 years for SV Peg(1990 -- 1995, 2000 -- 2007). In addition, maps were obtained of RX Boo withthe Very Large Array over several years. We find that most of the emission inthe circumstellar shell of RX Boo is located in an incomplete ring with aninner radius of 91 mas (15 AU). A velocity gradient is found in a NW--SEdirection. The maser region can be modelled as a shell with a thickness of 22AU, which is only partially filled. The gas crossing time is 16.5 years. Thering-like structure and the velocity gradient remained stable for at least 11years, while the maser line profiles varied strongly. This suggests that thespatial asymmetry is not accidental, so that either the mass loss process orthe maser excitation conditions in RX Boo are not spherically symmetric. Thestrong variability of the maser spectral features is mainly due to incoherentintensity fluctuations of maser emission spots, which have lifetimes of theorder of 1 year. We found no correlation between the optical and the maservariability in either star. The variability properties of the SV Peg masers donot differ substantially from those of RX Boo. There were fewer spectralfeatures present, and the range of variations was narrower. The maser wasactive on the >10-Jy level only 1990 -- 1992 and 2006/2007. At other times themaser was either absent (<1 Jy) or barely detectable.

Water vapour masers in long-period variable stars. I. RX Boo and SV Peg