Size gap for zero temperature black holes in semiclassical gravity

We show that a gap exists in the allowed sizes of all zero-temperature static spherically symmetric black holes in semiclassical gravity when only conformally invariant fields are present. The result holds for both charged and uncharged black holes. By size we mean the proper area of the event horizon. The range of sizes that do not occur depends on the numbers and types of quantized fields that are present. We also derive some general properties that zero and nonzero temperature black holes have in all classical and semiclassical metric theories of gravity. [S0556-2821(99)06604-7].