  We wish to understand the reasons for the strong variability of H2O masers in
circumstellar shells of late-type stars. We monitored RX Boo and SV Peg in the
22-GHz maser line of water vapour with single-dish telescopes. The monitoring
period 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 with
the Very Large Array over several years. We find that most of the emission in
the circumstellar shell of RX Boo is located in an incomplete ring with an
inner radius of 91 mas (15 AU). A velocity gradient is found in a NW--SE
direction. The maser region can be modelled as a shell with a thickness of 22
AU, which is only partially filled. The gas crossing time is 16.5 years. The
ring-like structure and the velocity gradient remained stable for at least 11
years, while the maser line profiles varied strongly. This suggests that the
spatial asymmetry is not accidental, so that either the mass loss process or
the maser excitation conditions in RX Boo are not spherically symmetric. The
strong variability of the maser spectral features is mainly due to incoherent
intensity fluctuations of maser emission spots, which have lifetimes of the
order of 1 year. We found no correlation between the optical and the maser
variability in either star. The variability properties of the SV Peg masers do
not differ substantially from those of RX Boo. There were fewer spectral
features present, and the range of variations was narrower. The maser was
active on the >10-Jy level only 1990 -- 1992 and 2006/2007. At other times the
maser was either absent (<1 Jy) or barely detectable.
