  We show that the central charges that group theory allows in the (2,0)
supersymmetry translations algebra arise from a string and a 3-brane by
commuting two supercharges. We show that the net force between two such
parallel strings vanishes. We show that all the coupling constants are fixed
numbers, due to supersymmetry, and self-duality of the three-form field
strength. We obtain a charge quantization for the self-dual field strength, and
show that when compactifying on a two-torus, it reduces to the usual
quantization condition of N=4 SYM with gauge group SU(2), and with coupling
constant and theta angle given by the tau-parameter of the two-torus, provided
that we pick that chiral theory which corresponds to a theta function with zero
characteristics, as expected on manifolds of this form.
