  Very Long Baseline Interferometry (VLBI) allows for high-resolution and
high-sensitivity observations of relativistic jets, that can reveal
periodicities of several years in their structure. We perform an analysis of
long-term VLBI data of the quasar S5 1928+738 in terms of a geometric model of
a helical structure projected onto the plane of the sky. We monitor the
direction of the jet axis through its inclination and position angles. We
decompose the variation of the inclination of the inner 2 milliarcseconds of
the jet of S5 1928+738 into a periodic term with amplitude of ~0.89 deg and a
linear decreasing trend with rate of ~0.05 deg/yr. We also decompose the
variation of the position angle into a periodic term with amplitude of ~3.39
deg and a linear increasing trend with rate of ~0.24 deg/yr. We interpret the
periodic components as arising from the orbital motion of a binary black hole
inspiraling at the jet base and derive corrected values of the mass ratio and
separation from the accumulated 18 years of VLBI data. Then we identify the
linear trends in the variations as due to the slow reorientation of the spin of
the jet emitter black hole induced by the spin-orbit precession and we
determine the precession period T_SO=4852+/-646 yr of the more massive black
hole, acting as the jet emitter. Our study provides indications, for the first
time from VLBI jet kinematics, for the spinning nature of the jet-emitting
black hole.
