  The long-term variability of the multiwavelength blazar emission can be
interpreted in terms of orientation variations of a helical, inhomogeneous,
non-thermally emitting jet, possibly caused by the orbital motion of the parent
black hole in a binary system (Villata & Raiteri 1999). The helical-jet model
is here applied to explain the quasi-periodic radio-optical light curves and
the broad-band spectral energy distributions (SEDs) of the BL Lac object AO
0235+16. Through a suitable choice of the model parameters, the helix rotation
can well account for the periodicity of the main radio and optical outbursts
and for the corresponding SED variability, while the interspersed minor radio
events could be interpreted as due either to some local distortions of the
helical structure or to other phenomena contributing to the source emission. In
particular, the probable existence of flow instabilities provides a viable
interpretation for the non-periodic features.
