Remarks on twisted theories with matter

We investigate some aspects of N = 2 twisted theories with matter hypermult iplets in the fundamental representation of the gauge group. A consistent f ormulation 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(2l). We obtain the blowup formula and show that the blowup function is g iven by a hyperelliptic sigma -function with singular characteristic. We co mpute the contact terms and find, as a corollary, interesting identities be tween hyperelliptic Theta functions.