Optical and radio behaviour of the blazer S4 0954+65

We present the results of an intensive optical and radio monitoring of the gamma-loud blazar S4 0954+65 from November 1994 to May 1998. The optical ob servations were done at the Torino, Perugia, and Tuorla Astronomical Observ atories; in this period, including two pointings by the Energetic Gamma Ray Experiment Telescope (EGRET), the source was very active and reached its h istorical maximum. Intranight variability was checked during several nights , giving a positive answer. Colour-index variations were also detected, but no definite trend with brightness can be recognized. The radio light curve s at 22 and 37 GHz from the Metsahovi Radio Observatory and those at 4.8 an d 14.5 GHz from the University of Michigan Radio Astronomy Observatory (UMR AO) are not well sampled; however, two interesting fast-variability events are present, from which brightness temperatures exceeding the Compton limit by several orders of magnitude can be calculated. A comparison of the opti cal data with the radio ones does not lead to meaningful results because of the poor radio sampling. Finally, we analyse the spectral energy distribut ions (SEDs) from the radio to the optical band using contemporaneous radio and optical data taken during two different optical states, with a low radi o flux corresponding to the high optical state and vice versa. We first fit these SEDs by means of a helical-jet model, which suggests that the observ ed large, long-term variations can be due to changes of the jet orientation with respect to the line of sight. Then we fit the same SEDs with a homoge neous model, which can account for the high-energy part of the SED, stressi ng that external soft photons in addition to the local ones are required to explain the gamma-ray data.