Electrically charged cold black holes in scalar-tensor theories

We study the possible existence of charged black holes in the Bergmann-Wago ner class of scalar-tensor theories (STT) of gravity in four dimensions. Th e existence of black holes is shown for anomalous versions of these theorie s, with a negative kinetic term in the Lagrangian. The Hawking temperature T-H of these holes is zero, while the horizon area is (in most cases) infin ite. As a special case, the Brans-Dicke theory is studied in more detail, a nd two kinds of infinite-area black holes are revealed, with finite and inf inite proper time needed for an infalling particle to reach the horizon; am ong them, analyticity properties select a discrete subfamily of solutions, parametrized by two integers, which admit an extension beyond the horizon. The causal structure and stability of these solutions with respect to small radial perturbations is discussed. As a by-product, the stability properti es of all spherically symmetric electrovacuum STT solutions are outlined.