Variability of the H2O maser associated with U Orionis

H2O line observations at lambda = 1.35 cm of the Mira Ceti-type variable star U Ori are reported. The observations cover the time interval from March, 1980, to September, 1999, Variations of the integral flux and velocity centroid of the H2O line are analysed. The flux in general correlates with the visual light curve, following it with some phase delay Delta phi similar to 0.2 - 0.4P (P is the period of the star). The maser emission is generated in a quasistationary layer of gas and dust at a distance of about 10(14) cm from the stellar centre, The maser variability is explained by the action of periodic shocks, driven by stellar pulsation and arriving to the maser in each stellar cycle. The shocks provide the maser Dumping, whereas the sink of the waste energy is controlled by the dust, periodically heated by the stellar radiation near the light maximum; this accounts for the correlation of tho maser radiation maximum with the descending branch of the light curve. Temporary weakness of the maser emission may be due to decay of the quasi-stationary layer, which is then rebuilt LS a powerful shuck, carrying an ay from the star a portion of the lost mass, once per a few stellar periods - the "superperiod". Ill its turn, the superperiod may reflect multiperiodic pulsation of tie star or the presence of a long-term activity cycle, connected with restructuring of the stellar magnetic field, which is known to be strong in U Ori.