  Given a unital C*-algebra A, an injective endomorphism \alpha:A --> A
preserving the unit, and a conditional expectation E from A to the range of
\alpha we consider the crossed-product of A by \alpha relative to the transfer
operator L=\alpha^{-1}E. When E is of index-finite type we show that there
exists a conditional expectation G from the crossed-product to A which is
unique under certain hypothesis. We define a "gauge action" on the
crossed-product algebra in terms of a central positive element h and study its
KMS states. The main result is: if h>1 and E(ab)=E(ba) for all a,b in A (e.g.
when A is commutative) then the KMS_\beta states are precisely those of the
form \psi = \phi G, where \phi is a trace on A satisfying the identity \phi(a)
= \phi(L(h^{-\beta}ind(E)a)), where ind(E) is the Jones-Kosaki-Watatani index
of E.
