2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/194974For a superstring theory in four spacetime dimensions, we propose a modification of the Born-Infeld action that possesses a well-defined tensionless limit. We interpret this as describing the effective target space dynamics of null strings on a D3-brane. We argue that such a modification can be induced by nonperturbative contributions from instantons in the worldsheet sigma-model describing string propagation on the brane.11 pages; Comments and references addedHigh Energy Physics - Theorytext