Curvature, Cones, and Characteristic Numbers
  We study Einstein metrics on smooth compact 4-manifolds with an edge-cone singularity of specified cone angle along an embedded 2-manifold. To do so, we first derive modified versions of the Gauss-Bonnet and signature theorems for arbitrary Riemannian 4-manifolds with edge-cone singularities, and then show that these yield non-trivial obstructions in the Einstein case. We then use these integral formulae to obtain interesting information regarding gravitational instantons which arise as limits of such edge-cone manifolds. 
Michael Atiyah and Claude LeBrun
math.DG