  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 ordering of operators. This
elucidates the mechanism that binds D-particles to an orientifold plane, in
accordance with the predictions of heterotic/type I duality. The ocurrence of
enhanced symmetries associated with massless bound states of a D-particle with
one orientifold plane is illustrated by the enhancement of $SO(14) \times U(1)$
to $E_8$ and $SO(12)\times U(1)$ to $E_7$ at strong type I' coupling.
Enhancement to higher-rank groups involves both orientifold planes. For
example, the enhanced $E_8 \times E_8 \times SU(2)$ 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 coincidence 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-engineered gauge theories is derived by relating it through
a chain of dualities to the Pauli exclusion principle.
