2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/202844We present some consequences of non-anomalous propagation requirements on various massless fields. Among the models of nonlinear electrodynamics we show that only Maxwell and Born-Infeld also obey duality invariance. Separately we show that, for actions depending only on the $F_\mn^2$ invariant, the permitted models have $L \sim \sqrt{1 + F^2}$. We also characterize acceptable vector-scalar systems. Finally we find that wide classes of gravity models share with Einstein the null nature of their characteristic surfaces.11 pages, LaTeX, no figuresHigh Energy Physics - TheoryGeneral Relativity and Quantum Cosmologytext