  The production of slow nucleons in semi-inclusive deep inelastic electron
scattering off the deuteron is investigated in the region $x \gsim 0.3$ for
kinematical conditions accessible at $HERA$. Within the spectator mechanism the
semi-inclusive cross section exhibits a scaling property, which can be used as
a model-independent test of the dominance of the spectator mechanism itself,
providing in this way an interesting tool to investigate the neutron structure
function. The possibility of extracting model-independent information on the
neutron to proton structure function ratio from semi-inclusive experiments is
also illustrated.
