  Exact solutions of Einstein equations with null Riemman-Christoffel curvature
tensor everywhere, except on a hypersurface, are studied using quantum
particles obeying the Klein-Gordon equation. We consider the particular cases
when the curvature is represented by a Dirac delta function with support either
on a sphere or on a cylinder (spherical and cylindrical shells). In particular,
we analyze the necessity of extra boundary conditions on the shells.
