Supersymmetric black holes in N=8 supergravity

We study the embedding of extreme (multi-) dilaton black hole solutions for the values of the parameter a = root 3, 1,1/root 3, 0 in N = 4 and N = 8 four-dimensional supergravity. For each black hole solution we find different embeddings in N = 4 supergravity which have different numbers of unbroken supersymmetries. When embedded in N = 8 supergravity, all different embeddings of the same solution have the same number of unbroken supersymmetries. Thus, there is a relation between the value of the parameter a and the number of unbroken supersymmetries in N = 8 supergravity, but not in N = 4, and the different embeddings must be related by dualities of the N = 8 theory which are not dualities of the N = 4 theory. The only exception in this scheme is a dyonic embedding of the a = 0 black-hole solution which seems to break all supersymmetries both in the N = 4 and in the N = 8 theories.