Boundary fixed points, enhanced gauge symmetry and singular bundles on K3

We investigate certain fixed points in the boundary conformal field theory representation of type IIA D-branes on Gepner points of K3. They correspond geometrically to degenerate brane configurations, and physically lead to e nhanced gauge symmetries on the world-volume. Non-abelian gauge groups aris e if the stabilizer group of the fixed points is realized projectively, whi ch is similar to D-branes on orbifolds with discrete torsion. Moreover, the fixed point boundary states can be resolved into several irreducible compo nents. These correspond to bound states at threshold and can be viewed as ( non-locally free) sub-sheaves of semi-stable sheaves. Thus, the BCFT fixed points appear to carry two-fold geometrical information: on the one hand th ey probe the boundary of the instanton moduli space on K3, on the other han d they probe discrete torsion in D-geometry. (C) 2001 Elsevier Science B.V. All rights reserved.