  We discuss the transformation properties of classical extremal N=2 black hole
solutions in S-T-U like models under S and T duality. Using invariants of
(subgroups of) the triality group, which is the symmetry group of the classical
BPS mass formula, the transformation properties of the moduli on the event
horizon and of the entropy under these transformations become manifest. We also
comment on quantum corrections and we make a conjecture for the one-loop
corrected entropy.
