  A two-dimensional Ising system with ferromagnetic coupling and one defect
line at distance L from the surface is solved exactly using Pfaffians. The
system shows a singularity in the surface correlation length at a temperature
T_s which is smaller than the transition temperature T_c of the bulk. Numerical
studies using the transfer matrix technique suggest that this singularity is
also present in an Ising system with two defect lines at distance L.
