The OH radio lines at 18 cm were monitored in six comets with the Nancay radio telescope in 1992 and early 1993: Zanotta-Brewington 1991g1, Bradfield 1992b, Mueller 1991h1, Shoemaker Levy 1991a1, P Swift-Tuttle 1992t and P/Schaumasse 1992x. Neither Bradfield 1992b nor Mueller 1991h1 could be detected. Zanotta-Brewington 1991g1, Shoemaker Levy 1991a1 and P/Schaumasse 1992x were relatively weak comets with OH production rates of the order of 10(28) s-1 and required several hours of integration to be detected. P/Swift- Tuttle was observed and detected as early as mid-October 1992. The signal was much stronger than for the other comets, especially at the beginning of December 1992, when P/Swift Tuttle was observed against galactic background radiation. The OH production rate was several 10(29) s-1. All along the observations, the OH radio lines exhibited unusually high blueshifts, corresponding to gas bulk motions of 0.5-1.0 km s-1 towards the observer. This undoubtedly corresponds to the strong jets observed in optical images and to the important nongravitational forces that affect the orbit of this comet. P/Swift-Tuttle was still observed at Nancay at the beginning of January 1993 when the small solar elongation precluded optical observations. A summary of these observations and of the estimations of the OH production rates is presented.
