  The phase transition in the antiferromagnetic isotropic Heisenberg S=1 chain
with frustrating next-nearest neighbor coupling alpha is reconsidered. We
identify the order parameter of the large-alpha phase as describing two
intertwined strings, each possessing a usual string order. The transition has a
topological nature determined by the change in the string connectivity.
Numerical evidence from the DMRG results is supported by the effective theory
based on soliton states.
