  A singular configuration of an external static vector field in the form of a
magnetic string polarizes the vacuum of a second-quantized theory on the plane
orthogonal to the string axis. The most general boundary conditions at the
punctured singular point that are compatible with the self-adjointness of
two-dimensional Dirac Hamiltonian are considered. The dependences of the
induced vacuum quantum numbers on the parameter of the self-adjoint extension,
on the string flux, and on the choice of irreducible representation of the
matrices in (2+1)-dimensional spacetime are disscussed.
