The non-perturbative SO(32) heterotic string

The SO(32) heterotic string can be obtained from the type IIB string by gauging a discrete symmetry that acts as(-1)(FL) on the perturbative string states and reverses the parity of the D-string. Consistency requires the presence of 32 NS 9-branes - the S-duals of D9-branes - which give SO(32) Chan-Paton factors to open D-strings. At finite string coupling, there are SO(32) charges tethered to the heterotic string world-sheet by open D-strings. At zero-coupling, the D-string tension becomes infinite and the SO(32) charges are pulled onto the world-sheet, and give the usual SO(32) world-sheet currents of the heterotic string. (C) 1999 Published by Elsevier Science B.V. All rights reserved.