  Nordstrom's theory of gravity, which describes gravity by a scalar field in
flat spacetime, is observationally ruled out. It is however the only theory of
gravity with General Relativity to obey the strong equivalence principle. I
show in this paper that this remarkable property is true beyond post-newtonian
level and can be related to the existence of a 'Nordstrom-Katz' superpotential.
