  Context. The Galactic microquasar SS 433 launches oppositely directed jets at
speeds approximately a quarter of the speed of light. Both the speed and
direction of the jets exhibit small fluctuations. A component of the speed
variation has 13 day periodicity and the orbital phase at which its maximum
speed occurs has advanced approximately 90 degrees in 25 years. Aims. To
examine the possibility that these variations are associated with a mildly
eccentric orbit and conditions necessary to achieve this apsidal advance.
Methods. The advance of the orbital phase for maximum speed is taken to be
advance of the apses of the putative elliptical orbit. It is compared with
calculations of the effects of tides induced in the companion and also with
gravitational perturbations from the circumbinary disc. These calculations are
made in the light of recent results on the SS 433 system. Results. The 13 day
periodicity in the speed of the jets of SS 433 might be attributed to a mildly
elliptical orbit, through periodic approaches of the donor and the compact
object. Advance of the apses of such an elliptical orbit due to tidal effects
induced in a normal companion looks to be to small; if caused by the
circumbinary disc the mass of the inner regions of that disc is ~ 0.15 solar
masses.
