1 Introduction

U Orionis is one of the brightest long-period variables belonging to the Mira Ceti type. According to GCVS (Kholopov et al. 1985) its period is 368.3$^{\rm d}$, the extreme limits of its visual brightness variations are $4.8-13.0^{\rm m}$, the spectral type varies between M6e and M9.5e. It has a rather asymmetric light curve with a steep ascending branch, (M-m)/P=0.38. The revised light elements (Kudashkina 1989) are

$${\rm Max} = 2445275.20+369.0^{\rm d}E.$$

U Ori is known to be a source of maser radio emission of OH (Wilson & Barrett 1970), H₂O (Wilson et al. 1972) and SiO (Kaifu et al. 1975). The star also displays thermal lines of CO (Knapp et al. 1998); the mass loss rate, determined from the thermal CO line, is $2.9~10^{-7}M_\odot$yr^-1, the star's radial velocity $V_{\rm LSR}=-38.1\pm1.3$ km s^-1. A recent distance estimate for U Ori, using the $P-M_{\rm K}$ dependence and the Hipparcos data, is $\sim300$ pc (Knapp et al. 1998).

The molecular maser emission of U Ori is highly variable, with peculiar changes, sometimes vanishing in some lines. In particular, in mid-1970s U Ori changed its type of the OH emission from IIb to I (Cimerman 1979; Jewell et al. 1979, 1981). In 1980 U Ori experienced a strong flare in the H₂O line $\lambda=1.35$ cm (Lekht et al. 1981), the flare lasted almost 1 year. All this indicates that the star may had been in a peculiar state, which is reflected in nonstationary processes in its circumstellar envelope, resulting in strong variations of the maser emission.

U Ori has been repeatedly studied by interferometry in the maser lines of OH (Reid et al. 1979; Fix et al. 1980; Bowers et al. 1981; Chapman & Cohen 1985; Bowers & Johnston 1988; Chapman et al. 1991) and H₂O (Lada et al. 1981; Bowers & Johnston 1994).

Figure 1: The profiles of the H2O line emission of the star U Ori. Vertical scale (flux density in Janskys) is common to all the graphs on the same panel. Horizontal axis is the radial velocity with respect to the Local Standard of Rest

Figure 1: continued

Figure 1: continued

Figure 1: continued

Figure 1: continued

U Ori is one of the Mira Ceti-type stars that are included into our program of long-term monitoring of H₂O maser sources (Berulis et al. 1984; Esipov et al. 1999). We have been observing U Ori in the 1.35-cm H₂O line since 1980. Some intermediate results on this star were published in Lekht et al. (1981), Berulis et al. (1983, 1984, 1987, 1990, 1994), where we discussed earlier studies of other authors on the H₂O maser variability of U Ori. In this paper we present the complete sample of all the H₂O spectra we obtained in 1980-1999, together with an analysis of the H₂O maser variability, comparison with the light curve of U Ori and with variations in other maser lines.