  General Relativity allows for a unique black hole solution, characterized by
its mass M, angular momentum J, and electric charge Q. Black holes in General
Relativity are thus said to have no hair, that is, no other independent
physical quantity (no-hair theorem). Despite the numerous successes of General
Relativity, some limitations remain, like the central singularity possessed by
black holes, where the curvature of spacetime becomes infinite. Modified
theories of gravity try to solve some of these shortcomings. This thesis tests
the no-hair theorem in a popular modification of gravity, called scalar-tensor
theories, where a unique degree of freedom (a scalar field) is added on top of
the usual metric of spacetime of General Relativity. Using various symmetries,
new black holes, called hairy black holes, are obtained. Some of them evade
strongly the no-hair theorem, being characterized by a new quantity (primary
scalar hair), distinct from M, J or Q. An interesting progress is also
achieved, since in certain cases, the usual singularity disappears: the
curvature of spacetime remains bounded even at the core of the black hole.
Moreover, theoretical links are established between scalar-tensor theories
(which take place in the usual four dimensions of spacetime), and theories of
gravity in higher dimensions, by Kaluza-Klein reduction. Finally, certain
particular properties of scalar-tensor theories enable to transform initial
black hole solutions into other solutions with very distinct geometry, like
wormholes or non-stationary black holes.
