  The one-dimensional free Fermi gas is a prototype conformally invariant
system, whose entanglement properties are well-understood. In this work, the
effects of a single impurity on one dimensional free fermion entanglement
entropy are studied both analytically and numerically. Such an impurity
represents an exactly marginal perturbation to the bulk conformally invariant
fixed point. We find that the impurity leads to sub-leading contributions to
the entanglement entropy that scale inversely with the subsystem size. The
origin of such contributions are identified.
