The group E₆ and the formulation of string theory

The group E₆ arises from the breaking of the E₈× E₈ symmetry of the heterotic string theory. It has a natural interpretation in terms of octonions and the formulation of string theory in terms of elements of this division algebras is noted. It is proven that a spinor bilinear can be formed which is invariant under E₈ and that the factorization of open superstring amplitudes may be extended to the exceptional group.