  We investigate some aspects of N=2 twisted theories with matter
hypermultiplets in the fundamental representation of the gauge group. A
consistent formulation of these theories on a general four-manifold requires
turning on a particular magnetic flux, which we write down explicitly in the
case of SU(2k). We obtain the blowup formula and show that the blowup function
is given by a hyperelliptic sigma-function with singular characteristic. We
compute the contact terms and find, as a corollary, interesting identities
between hyperelliptic Theta functions.
