Hadamard Renormalisation of the Stress Energy Tensor on the Horizons of   a Spherically Symmetric Black Hole Space-Time
  We consider a quantum field which is in a Hartle-Hawking state propagating in a general spherically symmetric black hole space-time. We make use of uniform approximations to the radial equation to calculate the components of the stress tensor, renormalized using the Hadamard form of the Green's function, on the horizons of this space-time. We then specialize these results to the case of the `lukewarm' Reissner-Nordstrom-de Sitter black hole and derive some conditions on the stress tensor for the regularity of the Hartle-Hawking state. 
Cormac Breen and Adrian C. Ottewill
gr-qc