D-Branes And K-Theory
  By exploiting recent arguments about stable nonsupersymmetric D-brane states, we argue that D-brane charge takes values in the K-theory of spacetime, as has been suspected before. In the process, we gain a new understanding of some novel objects proposed recently - such as the Type I zerobrane - and we describe some new objects - such as a -1-brane in Type I superstring theory. 
Edward Witten