Poincare invariance in effective string theories

We investigate the dispersion relation of the winding closed-string states in SU(N) gauge theory defined on a d-dimensional hypertorus, in a class of effective string theories. We show that order by order in the asymptotic expansion, each energy eigenstate satisfies a relativistic dispersion relation. This is illustrated in the Luscher-Weisz effective string theory to two-loop order, where the Polyakov loop matrix elements between the vacuum and the closed string states are obtained explicitly. We attempt a generalization of these considerations to the case of compact dimensions transverse to the string.