Semiregular variables, like RT Vir, are thought to be surrounded by circumstellar
gas-dust shells formed from the usual stellar mass loss. The 
maser
emission originates in the inner side of the shell at a distance of a few radii
from the upper photosphere (Lepine & Barros 1976). The maser emission has large
time variations which may correlate with stellar luminosity variations.
The semiregular variable RT Vir is an M 8 spectral type star, according to the
General Catalog of Variable Stars (Kholopov et al. 1985). The average period of
luminosity is 155 days and the magnitude varies between 
and
. Spencer et al. (1981) determined a distance to RT Vir of 1.35
kpc while Bowers et al. (1993) gave a distance of 700 pc. From SiO
(Spencer et
al. 1981) and 1667 MHz OH
(Le Squeren et al. 1970) line observations, a radial
velocity of the star equal to 
was deduced. Further observations
gave a value of 
(Nyman et al. 1986). The rate of mass loss is
(Bujarrabal et al. 1989).
The H2O maser emission from the gas-dust shell of RT Vir was first observed
by Dickinson (1976) in 1973 March. At that time, the profile had a single feature
with velocity of 
and flux of 16 Jy. Further observations
between 1975 and 1977 (Spencer et al. 1979;
Cox & Parker 1979) showed the
presence of a stronger component at 
, the flux of which was in the
range 100-300 Jy. The feature at 
did not exceed 30 Jy in those
observations.
Berulis et al. (1983) observed another feature of 
during
1980-1982. In this period the maser was not active. However, during
1984-1986
maser flares have taken place in a wide range of velocities
(Berulis
et al. 1987), probably caused by a shock wave. The amplitude of the strongest
flare attained 2400 Jy. Some spectral features underwent a velocity drift, which
was correlated with flux variations.
VLA observations of RT Vir have been reported for two epochs: 1985.05
(Bowers et
al. 1993) and 1988.95
(Bowers & Johnston 1994). The spatial distribution of
components with velocities between 12 and 
obtained by
Bowers &
Johnston (1994) was highly asymmetric relative to the star position and showed a
loop structure, which differs from the distribution found earlier
(Bowers et al.
1993).
Submillimetre maser emission from RT Vir at 321 GHz (1029-936 transition
of ortho and 325 GHz (515-422 transition of para ) was
also reported (Yates & Cohen 1996).
In this paper we present the observations of 22 GHz maser emission in
the direction of the semiregular variable RT Vir made during 1985-1996.