  In the framework of the four-dimensional effective theory of heterotic
superstrings at low energies, which is considered as a generalised theory of
gravity, we search for new black hole solutions. The analytical expressions of
all the scalar fields of the effective theory is determined in a Kerr-Newman
black hole background and the evasion of the existing "no-hair" theorems in the
presence of higher curvature gravitational terms is demonstrated. New black
hole solutions, such as the Dilatonic and the Coloured black holes, are
determined by use of numerical integration and their properties are analysed.
Finally, the linear stability of Dilatonic black holes under small
time-dependent perturbations is exhibited through a semi-analytic method.
