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.