  We calculate, numerically, the low-lying spectrum of closed confining flux
tubes that carry flux in different representations of SU(N). We do so for SU(6)
at beta=171, where the calculated low-energy physics is very close to the
continuum limit and, in many respects, also close to N=infinity. We focus on
the adjoint, 84, 120, k=2A,2S and k=3A,3M,3S representations and provide
evidence that the corresponding flux tubes, albeit mostly unstable, do in fact
exist. We observe that the ground state of a flux tube with momentum along its
axis appears to be well defined in all cases and is well described by the
Nambu-Goto free string spectrum, all the way down to very small lengths, just
as it is for flux tubes carrying fundamental flux. Excited states, however,
typically show very much larger deviations from Nambu-Goto than the
corresponding excitations of fundamental flux tubes and, indeed, cannot be
extracted in many cases. We discuss whether what we are seeing here are
separate stringy and massive modes or simply large corrections to energy levels
that will become string-like at larger lengths.
