2 Observations

Observations were made with the Nançay radio telescope. The half-power beamwidth at 1.6 GHz was $3.5^{\prime}$ in $\alpha$by $18^{\prime}$ in $\delta$. A cooled dual channel receiver was used to obtain signals in left- and right-handed circular polarisations (LHC, RHC) or in two orthogonal linear (horizontal and vertical) polarisations simultaneously. The typical system temperature was about 45 K and the point source sensitivity was 1.1 Jy K^-1 at 0$^{\circ}$declination. The 1024 channel, 7-level digitisation autocorrelation spectrometer was employed, divided into two 512 lag channels or four 256 lag channels providing a velocity resolution of 0.07 or 0.14 km s^-1, respectively, for both 1665 and 1667 MHz mainlines. Data were acquired in frequency switching mode. Each observation lasted about one hour and a typical 3${\sigma}$ sensitivity was 0.20 Jy for the spectra obtained at 0.14 km s^-1 of spectral resolution.

Monitoring observations of R Crt, W Hya and RT Vir at the 1665 and 1667 MHz in both circular polarisations started in 1982 January, 1986 January and 1982 April, respectively and lasted until 1995 December. We searched for OH 1612 MHz emission towards all the three stars near OH mainline maxima. During the last two years of the monitoring program the targets were also observed in two orthogonal linear polarisations and the results obtained will appear in a separate paper. The flux density scale was established by comparison with observations of W12. It was accurate to within 18% before 1985, about 8% in the period from 1985 to 1993 October and better than 5% since 1993 October. During all the observations, real-time correction was performed for the acceleration of topocentric velocity with respect to the local standard of rest (LSR). The LSR velocity  $v_{\rm LSR}$ was determined with an accuracy better than 0.01 km s^-1. Through the paper we used the terms red-shifted and blue-shifted to describe the velocity relative to the systemic velocity $v_{\rm s}$.