ON THE ORIGIN OF THE VARIABILITY OF SUPERLUMINAL RADIO-SOURCES SIMILAR TO 3C-273

The radio source 3C 273 shows phases of quasiperiodic activity and eje ctions of VLBI components on helical paths close to the nucleus. These characteristics can be explained, in the frame of the two-fluid model , by a precession phase of the jet followed by a long phase of ejectio n with a constant angle. During the precession phase a new e(+/-) comp onent is ejected relativistically and moves along the helical magnetic field lines. This new e(+/-) component is responsible for the gamma-r ay variability and the quasi-periodic light curve observed in the near IR and the optical. Then, this activity phase is followed by the appa rition of a new mm VLBI component after about one month when the compo nent becomes transparent to mm synchrotron radiation. The path of this new component is helical close to the nucleus and appears mostly line ar when the component escapes the perturbed region.