Discrete and continuous Yang-Mills measure for non-trivial bundles over   compact surfaces
  We construct one Yang-Mills measure on a compact surface for each isomorphism class of principal bundles over this surface. For this, we define a new discrete gauge theory which is essentially a covering of the usual one. We prove that the measures correponding to different isomorphism classes of bundles or to different total areas of the surface are mutually singular. We give also a combinatorial computation of the partition functions based on the formalism of fat graphs. 
Thierry Levy (DMA)