  In the context of four-dimensional SU(N) gauge theories, we study the
spectrum of the confining strings. We compute, for the SU(6) gauge theory
formulated on a lattice, the three independent string tensions sigma_k related
to sources with Z_N charge k=1,2,3, using Monte Carlo simulations. Our results,
whose uncertainty is approximately 2% for k=2 and 4% for k=3, are consistent
with the sine formula sigma_k/sigma = sin(k pi/N) / \sin(pi/N) for the ratio
between sigma_k and the standard string tension sigma, and show deviations from
the Casimir scaling.
  The sine formula is known to emerge in supersymmetric SU(N) gauge theories
and in M-theory. We comment on an analogous behavior exhibited by
two-dimensional SU(N)xSU(N) chiral models.
