Preservation of extreme points

We characterize the extreme points of the closed unit ball of the dual of a Banach space which are preserved by the adjoint of any extreme operator. The result is related to the structure topology introduced by Alfsen and Effros on the set of all extreme points in the dual of any Banach space. As a consequence, we prove that c0(I) is the only Banach space such that the adjoint of every extreme operator taking values into it preserves extreme points