5 Conclusions

We have monitored the OH maser emission of three semiregular variables with the Nançay radio telescope over intervals spanning 10-14 years. This study leads to the following conclusions:

1. The OH masers show cyclic variations with slow high amplitude ( $4{-}6^{\rm m}$) rises and declines, superimposed with fast low amplitude ( $0.3{-}2^{\rm m}$) changes. The OH curves of W Hya follow optical variations, which is expected for radiative pumping. The variations of the OH masers from R Crt and RT Vir are less regular and the inferred radio periods are inconsistent with the optical periods. Erratic behaviour of these OH masers can be due to transient instabilities in hotter thinner envelopes. Well-sampled optical data will be important to check a possible radiative coupling of the OH mainline masers with the stellar variability.

2. There is evidence for the presence of an outer detached OH shell around W Hya with a radius of over 10¹⁶ cm. Emission at 1667 MHz at extreme velocities appears erratically, and its variability profile compared with other parts of the shell and with interferometric observations suggests it is at distances typical of a 1612 MHz shell. Emission at this frequency has been detected once only. OH emission from the outer shell appears to sporadically excited at times of general high OH activity.

3. All studied stars sometimes show a weak emission near the systemic velocity which is due to tangential amplification. This emission usually appears during a high level of the total OH emission which is predominantly radially beamed. For R Crt, we found that the tangential emission decreased below our sensitivity limit when the kinetic temperature in the maser envelope drops below 150-200 K.

4. We observed bursts of the 1667 MHz features in RT Vir and W Hya. The line narrowing and re-broadening on timescales of 90-200 days are well documented for 3 features. The inverse relationship between the linewidth and the peak flux density established for those features suggests variations in the maser gain during unsaturated amplification.

5. The ratios of the flux density at 1667 MHz over that at 1665 MHz in all the three stars were about 2 at epochs of high OH activity and usually increased during weak maser emission. This finding seems to support the theoretical prediction that the line ratio is a function of the fractional abundance of OH molecules.

6. Simultaneous single dish and interferometric observations at high angular resolution of these and other maser transitions which probe different circumstellar regions and regular optical observations would provide useful data to extend our findings.

Acknowledgements

We would like to thank Anita Richards for helpful suggestions on the manuscript. S. E. thanks the "Société de Secours des Amis des Sciences'' for financial support through a research grant. The Unité Scientifique Nançay of the Observatoire de Paris is associated as Unité de Service et de Recherche USR No. B704 to the French Centre National de Recherche Scientifique (CNRS).