Unbinding of three strings in the Born-Oppenheimer approximation

The unbinding transition of three strings, fluctuating without overhangs in two dimensions, is considered in the Born-Oppenheimer approximation. The calculation, which is extremely simple, gives some insight into the physical considerations that determine the universality class. In the case of short-range pair interactions the unbinding transition is predicted to be universal, with the same critical exponents as in the unbinding of two strings. For strings bound by short-range three-body interactions the unbinding transition is nonuniversal. Both of these predictions are in agreement with recent exact studies.Dedicated to H. Wagner