  We study the spectrum of the confining strings in four-dimensional SU(N)
gauge theories. We compute, for the SU(4) and SU(6) gauge theories formulated
on a lattice, the string tensions sigma_k related to sources with Z_N charge k,
using Monte Carlo simulations. Our results 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.
  For the SU(4) and SU(6) cases the accuracy is approximately 1% and 2%,
respectively. The sine formula is known to emerge in various realizations of
supersymmetric SU(N) gauge theories. On the other hand, our results show
deviations from Casimir scaling. We also discuss an analogous behavior
exhibited by two-dimensional SU(N) x SU(N) chiral models.
