In this paper we describe the Seiberg-Witten invariants, which have been
introduced by Witten, for manifolds with b+=1. In this case the invariants
depend on a chamber structure, and there exists a universal wall crossing
formula. For every K\"ahler surface with pg=0 and q=0, these invariants
are non-trivial for all Spinc(4)-structures of non-negative index.Comment: LaTeX, 27 pages. To appear in Int. J. Mat