(8,0) QUANTUM-MECHANICS AND SYMMETRY ENHANCEMENT IN TYPE I' SUPERSTRINGS

The low-energy supersymmetric quantum mechanics describing D-particles in the background of D8-branes and orientifold planes is analyzed in detail, including a careful discussion of Gauss' law and normal orderi ng of operators. This elucidates the mechanism that binds D-particles to an orientifold plane, in accordance with the predictions of heterot ic/type I duality. The ocurrence of enhanced symmetries associated wit h massless bound states of a D-particle with one orientifold plane is illustrated by the enhancement of SO(14) x U(1) to E-8 and SO(12) to E -7 at strong type I' coupling. Enhancement to higher-rank groups invol ves both orientifold planes. For example, the enhanced E-8 x E-8 x SU( 2) symmetry at the self-dual radius of the heterotic string is seen as the result of two D8-branes coinciding midway between the orientifold planes, while the enhanced SU(18) symmetry results from the coinciden ce of all sixteen D8-branes and SO(34) when they also coincide with an orientifold plane. As a separate by-product, the s-rule of brane-engi neered gauge theories is derived by relating it through a chain of dua lities to the Pauli exclusion principle.