Classical Entropy of N=2 Black Holes: The minimal coupling case
  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. 
W. A. Sabra