We discuss the entropy and the transformation properties of classical
extremal N=2 black hole solutions in supergravity theories associated with the
minimal coupling models CP(n-1,1). The entropy is given by a manifestly
invariant quantity under the embedding of the duality group SU(1,n) into
Sp(2n+2) which is a symmetry of the classical BPS mass formula.Comment: 8 pages, Late