  In three dimensions there is a logarithmically divergent contribution to the
entanglement entropy which is due to the vertices located at the boundary of
the region considered. In this work we find the corresponding universal
coefficient for a free Dirac field, and extend a previous work in which the
scalar case was treated. The problem is equivalent to find the conformal
anomaly in three dimensional space where multiplicative boundary conditions for
the field are imposed on a plane angular sector. As an intermediate step of the
calculation we compute the trace of the Green function of a massive Dirac field
in a two dimensional sphere with boundary conditions imposed on a segment of a
great circle.
