A ug 2 00 7 Scalar - tensor black holes coupled to Euler - Heisenberg nonlinear electrodynamics

The no-scalar-hair conjecture rules out the existence of asymptotically flat black holes with a scalar dressing for a large class of theories. No-scalar-hair theorems have been proved for the cases of neutral black holes and for charged black holes in the Maxwell electrodynamics. These theorems, however, do not apply in the case of non-linear electrodynamics. In the present work numerical solutions describing charged black holes coupled to Euler-Heisenberg type nonlinear electrodynamics in scalar-tensor theories of gravity with massless scalar field are found. In comparison to the corresponding solution in General Relativity the presented solution has a simpler causal structure the reason for which is the presence of the scalar field. The present class of black holes has a single, nondegenerate horizon, i.e., its causal structure resembles that of the Schwarzschild black hole. Pacs numbers: 4.25.Dm, 04.40.-b, 04.70.-s, 95.30.Tg, 97.60.Lf