TESTING EFFECTIVE STRING MODELS OF BLACK-HOLES WITH FIXED SCALARS

We solve the problem of mixing between the fixed scalar and metric flu ctuations. First, we derive the decoupled fixed scalar equation for th e four-dimensional black hole with two different charges. We proceed t o the five-dimensional black hole with different electric (one-brane) and magnetic (five-brane) charges, and derive two decoupled equations satisfied by appropriate mixtures of the original fixed scalar fields. The resulting greybody factors are proportional to those that follow from coupling to dimension (2,2) operators on the effective string. In general, however, the string action also contains couplings to chiral operators of dimension (1,3) and (3,1), which cause disagreements wit h the semiclassical absorption cross sections. Implications of this fo r the effective string models are discussed.