  Within the 'Medicina/Effelsberg H2O maser monitoring program' we have
observed the maser emission of R Crt and RT Vir for more than two decades. To
get insight in the distribution and longevity of maser spots in the
circumstellar envelopes, we have collected interferometric data, taken in the
same period, from the literature.
  We confirm short-time variations of individual maser features on timescales
of months to up to 1.5 years. Also decade-long variations of the general
brightness level independent from individual features were seen in both stars.
These are due to brightness variations occurring independently from each other
in selected velocity ranges, and are independent of the optical lightcurves.
Expected drifts in velocity of individual features are usually masked by
blending. However, in RT Vir we found an exceptional case of a feature with a
constant velocity over 7.5 years (<0.06 km/s/yr).
  We attribute the long-term brightness variations to the presence of regions
with higher-than-average density in the stellar wind, which host several clouds
which emit maser radiation on the short time scales. These regions typically
need ~20 years to cross the H2O maser shell, where the right conditions to
excite H2O masers are present. The constant velocity feature (11 km/s) is
likely to come from a single maser cloud, which moved through about half of RT
Vir's H2O maser shell without changing velocity. From this we infer that its
path was located in the outer part of the H2O maser shell, where RT Vir's
stellar wind apparently has already reached its terminal outflow velocity. This
conclusion is corroborated by the observation that the highest H2O maser
outflow velocity in RT Vir approaches the terminal outflow velocity as given by
OH and CO observations. This is generally not observed in other semi-regular
variable stars.
