  We study the entanglement properties of anisotropic open spin one-half
Heisenberg chains with a modified central bond. The entanglement entropy
between the two half-chains is calculated with the density-matrix
renormalization method (DMRG).We find a logarithmic behaviour with an effective
central charge c' varying with the length of the system. It flows to one in the
ferromagnetic region and to zero in the antiferromagnetic region of the model.
In the XX case it has a non-universal limit and we recover previous results.
