  We consider a conformal system of a string and a particle defined in D=10+2
space-time dimensions. The extra time-like dimension is a gauge artifact and
can be eliminated by choosing a gauge in which the SO(10,1) Lorentz symmetry is
manifest. The effective theory of string observables is the 11d supergravity.
The same theory compactified on T^2 provides a non-perturbative unified picture
of the Type IIA, Type IIB and 11d supergravity. This is confirmed by explicit
determination of the R^4-terms which are finite and manifestly SL(2,Z)
invariant as expected by the U-duality conjecture in nine non-compact
dimensions with maximal supersymmetry.

High Energy Physics - Theory