Monitoring of long term behaviour of OH masers in semiregular variables: R Crt, W Hya and RT Vir

We present and interpret the results of a long-term OH variability study of three semiregular stars, one type a (SRa), W Hya, and two type b (SRb), R Crt and RT Vir. The 1665 and 1667 MHz OH masers of the three semiregulars were observed at intervals in the period 1982 January-1995 December using the Nançay radio telescope, and we searched for 1612 MHz emission. The OH maser profiles of the studied stars significantly deviated from a standard double-peaked profile. The timescale of profile changes in the two SRb stars R Crt and RT Vir was as short as a month. The OH profiles of the SRa star W Hya were much more stable but since November 1986 a very blue-shifted feature appeared at 1667 MHz. Our phase-lag measurements suggest that this feature comes from a detached OH shell of radius ~$3\times 10^{16}$ cm. Faint 1612 MHz emission was found in W Hya only. Weak emission at velocities very close to the systemic velocity usually appeared during some intervals of high maser activity in R Crt and RT Vir and was almost always present in W Hya. For R Crt we estimated that this tangential emission disappeared when the kinetic temperature in the OH maser regions dropped below 150-200 K. For a few features, line narrowing and re-broadening were observed on timescales of 90-200 days. The linewidth was inversely proportional to the peak flux density, suggesting unsaturated amplification. Cyclic variations in the integrated flux density were observed in all the three stars. The OH variability curves were generally characterised by large amplitude ($4{-}6^{\rm m}$) variations over 400-800 days superimposed with 100-300-day variations of $0.2{-}2^{\rm m}$. Only the measured OH period of W Hya, of $362\pm 7$ days, was in agreement with the optical period. The two SRb stars exhibited multi-periodic OH variability including with two statistically significant periods. The behaviour of their red- and blue-shifted emission was less correlated than in W Hya. The ratios of the flux densities at 1667 MHz to 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. Long term behaviour of the OH masers from W Hya resembled that of standard OH Miras, while that of R Crt and RT Vir suggested thin and clumpy envelopes where unsaturated emission was sustained in some clouds