Vacuum polarization for lukewarm black holes

We compute the renormalized expectation value of the square of a quantum scalar field on a Reissner-Nordstr\"om--de Sitter black hole in which the temperatures of the event and cosmological horizons are equal (``lukewarm'' black hole). Our numerical calculations for a thermal state at the same temperature as the two horizons indicate that this renormalized expectation value is regular on both the event and cosmological horizons. We are able to show analytically, using an approximation for the field modes near the horizons, that this is indeed the case.