4 Conclusions

We have been monitoring the maser emission of the Mira Ceti-type variable star U Ori in the 1.35-cm H₂O line during 19 years (1980-1999). The star was quite active in the H₂O line in 1980-1988, with a giant flare that took place between June and October, 1980. Then the curve of the integrated H₂O flux variability of U Ori went on in the mode of damped oscillations and finally stabilised in the form of repeated outbursts to a level of 50-100 Jy km s^-1, following the visual light maxima of the star with some phase delay $\Delta\varphi$. The phase delay did not remain constant, varying from one variability cycle to another. These results confirm the main conclusion we drew earlier (Berulis et al. 1994) about quasi-periodic variations of $\Delta\varphi$ with a certain "superperiod'' of the star's activity, about 9 years long.

Once per each "superperiod'' the star sends a powerful shock, which consecutively excites emissions in the H$\alpha$, SiO, H₂O, and OH lines. Such a sequence of events was traced by us in the Mira R Leo (Esipov et al. 1999).

The star's "superperiod'' may reflect, on the one hand, the multiperiodicity of the pulsations themselves, namely a presence of a long period, found in the long time series of visual observations in some Miras (Percy & Bagby 1999), or, on the other hand, some kind of a long-term activity cycle, similar to the solar 11/22-year cycle; the latter possibility may be due to general stellar mass-loss variations, connected with restructuring of the stellar magnetic field, which is found to be quite strong in U Ori, up to 10 G near its surface (Reid et al. 1979; Fix 1979; Fix et al. 1980; Claussen & Fix 1982).

Acknowledgements

We thank the staff of the Pushchino Radio Astronomy Observatory for the great help with the H₂O observations. This work was supported by the Russian Foundation for Basic Research (project code 96-02-18867), Russian State Program in Astronomy and Fundamental Space Research (grant 1.4.3.1) and Russian Program of Integration of Science and Higher School (project 315). G.M.R. gratefully acknowledges the hospitality of INAOE at the time when this article was written. The authors are grateful to Dr. J.A. Mattei, the Director of the AAVSO, for the kindly supplied data of visual observations of U Ori. This research also made use of the AFOEV visual observations from the SIMBAD database, operated at Centre des données astronomiques de Strasbourg (France).