The 
profiles of the semiregular variable RT Vir exhibit fast and large
variations. In contrast with sources associated with star formation regions,
flares of the features in RT Vir lasting more than three months were not
observed. Series of flares with time delays between flares of different features
from one to eight months were observed (Berulis et al. 1987). This is in
agreement with the results of Engels et al. (1988), who also found that for
semiregular variables the individual spectral features may be stable over a
timescale of a few months. Hence the lifetime of the maser condensations at an
active stage may have certain time limits.
Nevertheless, the analysis of our spectra showed that some features may be
observed during periods of up to 1.5 years. Large velocity drifts of these
features, attaining 
, were observed. The results of a more
detailed analysis of the individual components of RT Vir will be
published later.
On the basis of the whole data one may suggest that variations with timescales of up to 1.5 years depend not only on variations of the temperature structure and conditions of pumping (Lewis & Engels 1991), but also on other factors. The region of maser generation located in the shell may be displaced in radial direction (Berulis et al. 1983), and in case of the existence of a velocity gradient of the gas in the shell a drift of the spectral features will be observed.
The existence of some stable regions of maser activity in RT Vir (Fig. 7 (click here)) shows that individual spectral features may be stable during a few years (Engels et al. 1988) not only in Mira variables but also in semiregular variables in the form of filaments or clumps distributed in the radial direction.
The long-term variation components of the integrated flux of two spectral parts (blue- and redshifted relative the star velocity) have a characteristic timescale of 5-6 years (Fig. 3 (click here)). The maxima of these curves are in opposite phase, i.e. there is anticorrealation of the fluxes. This anticorrelation may have a nature other than that of a Keplerian disc in S255 (Cesaroni 1990) and S140 (Lekht et al. 1993) or that of a circumstellar toroidal structure, where the matter is sufficiently fragmented, as in W31A and W75S (Lekht et al. 1995). In such structures, there is a competition of radiative spatial modes for the pumping. (The possibility of a mere coincidence may not be excluded, since for characteristic variation timescales of 5-6 years a time interval of 11 years is not enough for revealing more clearly the anticorrelation of the fluxes).
A common factor with a Keplerian disc and toroid is that the maser condensations,
responsible for emission at the right and left parts of the spectra are in
opposite sides of the RT Vir shell. For stars of late spectral types the
material outflow may be weakly bipolar (Bowers & Johnston 1994). In this case
the parts of the shell, which are most strongly influenced by the bipolar
outflow, have the most appropriate conditions for the maser emission.
If we assume that the intensities of the bipolar outflow change in opposite
phase, then the intensity of the maser emission of two groups of features,
located on opposite sides of the shell, will change, i.e. anticorrelation of
the integrated fluxes will be observed.
The more or less periodic character of the integrated flux variation with period of about 5-6 years considerably exceeds the period of optical variability of the star.