CONFORMAL-INVARIANCE OF BLACK-HOLE TEMPERATURE

It is shown that the surface gravity and temperature of a stationary b lack hole are invariant under conformal transformations of the metric that are the identity at infinity. More precisely, we find a conformal invariant definition of the surface gravity of a conformal Killing ho rizon that agrees with the usual definition(s) for a true Killing hori zon and is proportional to the temperature as defined by Hawking radia tion. This result is reconciled with the intimate relation between the trace anomaly and the Hawking effect, despite the non-invariance of t he trace anomaly under conformal transformations.