“…We have, at present, no specific interpretation of their role, although the 17 In the 10-d MN solution, the location of the Dirac string for the magnetic 2-form C 2 is specified by an angular variable ψ that can take 2N different values for the same field theory θ-angle, but the solution is symmetric under a shift by π of ψ, leaving N different configurations. Equivalently, one has N different ways of placing probe D5-branes in the background, in order to obtain the same field theory action [25,27,29].…”