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Summary: We study the non-perturbative Coulomb phase of $D=6$, $N=1$ heterotic string vacua with more than one tensor multiplet. We find that, for $n_T>9$, the Coulomb branch is divided by a codimension-one hypersurface, across which the coupling of gravitational curvature-squared term changes sign. We find a new class of BPS soliton strings whose core is supported by a gravitational instanton, i.e. a self-dual gravitational string. For $n_T>9$, tension of the string vanishes at a codimension-one hypersurface. We suggest that the tensionless gravitational string might be related to a novel infrared phase of quantum gravity.
