  Context: In the binary system LSI+61303 the peak flux density of the radio
outburst, which is related to the orbital period of 26.4960 +/- 0.0028d,
exibits a modulation of 1667 +/- 8 d. The radio emission at high spatial
resolution appears structured in a precessing jet with a precessional period of
27-28 d. Aims: How close is the precessional period of the radio jet to the
orbital period? Any periodicity in the radio emission should be revealed by
timing analysis. The aim of this work is to establish the accurate value of the
precessional period. Methods: We analyzed 6.7 years of the Green Bank
Interferometer database at 2.2 GHz and 8.3 GHz with the Lomb-Scargle and phase
dispersion minimization (PDM) methods and performed simulations. Results: The
periodograms show two periodicities, P1 = 26.49 +/- 0.07 d (\nu1=0.03775
d^{-1}) and P2 = 26.92 +/- 0.07 d (\nu2 = 0.03715 d^{-1}). Whereas radio
outbursts have been known to have nearly orbital occurrence P1 with timing
residuals exhibiting a puzzling sawtooth pattern, we probe in this paper that
they are actually periodical outbursts and that their period is Paverage=
(2/(\nu1 + \nu2)= 26.70 +/- 0.05 d. The period Paverage as well as the
long-term modulation Pbeat=1/(\nu1 - \nu2)=1667 +/- 393 d result from the beat
of the two close periods, the orbital P1 and the precessional P2 periods.
Conclusions: The precessional period, indicated by the astrometry to be of
27--28 d, is P2=26.92 d. The system \lsi seems to be one more case in astronomy
of beat, i.e., a phenomenon occurring when two physical processes create stable
variations of nearly equal frequencies. The very small difference in frequency
creates a long-term variation of period 1/(\nu1-\nu2). The long-term modulation
of 1667 d results from the beat of the two close orbital and precessional
rates.
