1 Introduction

OH maser emission in late-type stars was first detected by Wilson & Barrett (1968). This emission comes from the circumstellar shell of these objects and can be detected in three of the four predicted transitions: in the 1612 MHz satellite line and in both main lines at 1665 and 1667 MHz. Miras are OH maser variable late-type stars, and can be observed at various other wavelengths, in the optical and infrared ranges. Harvey et al. (1974) and Fillit et al. (1977) have shown that the variations of the emissions in the visible, infrared and OH wavelengths are correlated. In particular, the proposition of infrared OH radiative pumping coming from dust was motivated by the correlation between the infrared and OH variations. Moreover, allowed transitions of the OH molecule exist in the near and far infrared.

The periods of the cyclic variations of these OH stars range from about 300 to 600 days (Kukarkin et al. 1969). Thus, one of the main constraints on studies of their variations is that several years of observations are needed.

The general shapes of the optical, infrared and OH variation curves are rather similar. Especially, one can see asymmetry between the ascending and descending parts of the curve corresponding to a difference between the rise time toward a maximum and the fall time toward a minimum (Le Bertre 1993; Herman & Habing 1985; van Langevelde et al. 1990, 1993).

For some of these stars, cyclic ${\rm H_{2}O}$ maser emission can also be observed. Gómez Balboa & Lépine (1986) have shown that some aperiodic variability can be observed in these ${\rm H_{2}O}$ maser emissions which span several cycles of the stellar periodicity. In some cases, they even observed superperiodic behaviour with a period equal to a multiple of the fundamental stellar period.

The standard OH spectrum of Miras is characterized by a two-peaked profile in which the peaks are sometimes called "blue'' and "red'' (i.e., respectively blue-shifted and red-shifted with respect to the stellar velocity). According to the standard model, i.e., an envelope in radial expansion (Reid et al. 1977), these emissions come from the front and back part of the shell respectively. Nevertheless, sometimes one can observe an "intermediate" peak located between the two standard ones.

Except for eruptive Miras U Ori (Jewell et al. 1979, 1981; Cimerman 1979), X Oph (Etoka & Le Squeren 1996), R Leo, R Cnc, R LMi and U Her (Etoka & Le Squeren 1997), OH variability studies presented in the literature deal with only 2 or 3 consecutive cycles; variations over longer time spans have never been examined. The study presented here deals with observations in the 3 OH maser lines of 7 Miras performed over 15 years. These Miras are R Aql, RS Vir, S CrB, RR Aql, R LMi, U Her, and UX Cyg. The main aims of the study are to determine:

• the behaviour of the OH maser emission from one cycle to another;
• the longevity and variability behaviour of the various spectral components in each OH line;
• whether there are differences in the variations between standard and intermediate peaks.

In the next section we present the main characteristics of the observations. In Sect. 3 the methods of analysis used for this study are presented. In Sect. 4 the results concerning each star are presented separately. In Sect. 5 the main common characteristics of the various OH variations and their interpretation are given and, finally the conclusion of the study is given in Sect. 6.