2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/53942We describe how geometrical methods can be applied to a system with explicitly time-dependent second-class constraints so as to cast it in Hamiltonian form on its physical phase space. Examples of particular interest are systems which require time-dependent gauge fixing conditions in order to reduce them to their physical degrees of freedom. To illustrate our results we discuss the gauge-fixing of relativistic particles and strings moving in arbitrary background electromagnetic and antisymmetric tensor fields.8 pages, Plain TeX, CERN-TH.7392/94 and MPI-PhT/94-45High Energy Physics - TheoryGeneral Relativity and Quantum Cosmologytext