  Scalar-tensor (ST) gravity is considered in the case where the scalar is an
external field. We show that General Relativity (GR) and usual ST gravity are
particular cases of the External Scalar-Tensor (EST) gravity. It is shown with
a particular cosmological example that it is possible to join a part of a GR
solution to a part of a ST one such that the complete solution neither belongs
to GR nor to ST, but fully satisfies the EST field equations. We argue that
external fields may effectively work as a type of screening mechanism for ST
theories.
