2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/198776We study the space-time CPT properties of string theories formulated in a flat Minkowski background of even dimension. We define CPT as a world-sheet transformation acting on the vertex operators and we prove the CPT invariance of the string $S$-matrix elements. Some related issues, including the connection between spin and statistics of physical string states, are also considered.1+38 pages, macros includedHigh Energy Physics - Theorytext