Motivated by closed string perturbation theory arguments, I consider nonperturbative effects in the string coupling in supersymmetric critical heterotic string theories that are stronger than field theory ones. I will argue that in 10D, such effects arise from heterotic “D-instantons,” i.e. heterotic disk diagrams, whose existence relies on a non-trivial interplay between worldsheet and spacetime degrees of freedom.  Two general principles arise: The first is that the consistency of those heterotic branes on which the fundamental string can end relies on an inflow mechanism for spacetime degrees of freedom. The second is that the perturbation theory argument, taken to its logical conclusion, implies that all closed-string theories must exhibit open strings as well. Finally, I will show how these effects can have an impact on the vacuum energy of the 4D effective theory.