1 Introduction

Temporal variability is one of the characteristic properties of circumstellar OH masers. Early observations of Miras and OH/IR objects showed that OH maser emission follows the optical and infrared emission (Harvey et al. 1974; Fillit et al. 1977; Jewell et al. 1979). They show evidence for radiative coupling of the OH maser with stellar variability. Long term studies of Miras spanning about 15 years (Etoka 1996; Etoka & Le Squeren 2000) revealed that the integrated OH flux densities show cyclic variations superimposed on slow changes lasting several stellar cycles. There are some Mira variables which experienced eruptive variability in the OH lines (Jewell et al. 1979; Le Squeren & Sivagnanam 1985; Etoka & Le Squeren 1996; 1997).

In contrast to the extensively studied Mira variables, little is known about the OH variability of semiregular variables (SRs). It is commonly accepted that both classes of red giants are closely related; their periods are similar, but by definition the light curves of SRs are less regular than that of Miras and their optical amplitudes are less than 2 $.\!\!^{\rm m}$5 (Kholopov et al. 1985). The deduced mass loss rate of SRs are lower than that of Miras. An SRs-Miras evolutionary sequence was argued (Kerschbaum & Hron 1992). Young et al. (1993) suggested that some SRs had higher mass loss rates in the past implying that an opposite evolutionary sequence is allowed. The SRs associated with OH maser emission only form a small group; in the close solar neighbourhood about 10% of SRs show OH masers (Szymczak et al. 1995). Furthermore, the OH luminosity and the efficiency of OH pumping by the infrared emission in these stars are lower than those measured in giant stars with higher mass loss rates. The OH maser envelopes of the SRs are usually smaller and thinner than those observed in Miras and OH/IR stars (Szymczak et al. 1998, 1999). Those properties suggest that SRs can be good candidates to search for peculiar and/or non-cyclic variations in the OH maser flux density. A search for such behaviour of OH masers over several stellar cycles in three semiregular variables: R Crt, W Hya and RT Vir is one of the goals of our observations. We report the results of a monitoring program spanning 10-14 years, which duration is comparable to the gas travel time across the OH maser shells of the studied objects.

The observed intensity of OH maser sources can vary due to changes in the pump rate (Harvey et al. 1974; Le Squeren & Sivagnanam 1985) but changes within the maser regions due to shocks or disturbances propagating through the masing column may also be possible causes. OH monitoring data of SRs can provide new evidence which may be useful to verify the recent models of OH variations (Palen & Fix 2000) and OH mainline pumping (Collison & Nedoluha 1993). These monitorings also provide information to supplement high angular resolution images showing the spatial structure of the maser emission (Szymczak et al. 1998, 1999) and help us to understand better the dynamic properties of circumstellar outflow in OH maser envelopes.