THE unusual galactic object SS433 is well known because of the periodic red and blueshifts, corresponding to velocities of 50,000 kms-1, of some of its emission lines1,2. It is now believed to be a binary system that emits two oppositely directed precessing jets moving with a speed of 0.26c. The jets are produced and controlled by an accretion disk, probably geometrically thick, around a compact object whose nature is still controversial. Several arguments have been advanced21-23suggesting that it is a black hole. Here we report spectroscopic observations of the He II line at 4,686 Å from which we deduce a new estimate of the orbital speed of the compact object. Together with the mass ratio of the binary components, derived from X-ray observations, we find that the compact object is a neutron star, not a black hole. Analysis of the double-peaked profile of the He II line suggests that it is emitted by the accretion disk (or its corona), which is partly obscured by an opaque wind from the hot-spot region.