  Reflection walls of certain primitive vectors in the anti-invariant
sublattice of the K3 lattice define Heegner divisors in the period space of
Enriques surfaces. We show that depending on the norm of these primitive
vectors, these Heegner divisors are either irreducible or have two irreducible
components. The two components are obtained as the walls orthogonal to
primitive vectors of the same norm but of different type as ordinary or
characteristic.
