Tachyon condensation in unstable type-I D-brane systems

Type-I string theory provides eight classes of unstable D-brane systems. We determine the gauge group and tachyon spectrum for each one, and thereby d escribe the gauge symmetry breaking pattern in the low-energy world-volume field theory. The topologies of the resulting coset vacuum manifolds are re lated to the real K-theory groups KO-n, extending the known relations betwe en the type-II classifying spaces BU and U and the complex K-theory groups K-0 and K-1. We also comment on the role of the background D9-branes.