Resumen: We review and slightly improve the known k-polysymplectic Marsden–Weinstein reduction theory by removing some technical conditions on k-polysymplectic momentum maps by developing a theory of affine Lie group actions for k-polysymplectic momentum maps, removing the necessity of their co-adjoint equivariance. Then, we focus on the analysis of a particular case of k-polysymplectic manifolds, the so-called fibred ones, and we study their k-polysymplectic Marsden–Weinstein reductions. Previous results allow us to devise a k-polycosymplectic Marsden–Weinstein reduction theory, which represents one of our main results. Our findings are applied to study coupled vibrating strings and, more generally, k-polycosymplectic Hamiltonian systems with field symmetries. We show that k-polycosymplectic geometry can be understood as a particular type of k-polysymplectic geometry. Finally, a k-cosymplectic to ℓ-cosymplectic geometric reduction theory is presented, which reduces, geometrically, the space-time variables in a k-cosymplectic framework. An application of this latter result to a vibrating membrane with symmetries is given. Idioma: Inglés DOI: 10.1016/j.geomphys.2023.104899 Año: 2023 Publicado en: JOURNAL OF GEOMETRY AND PHYSICS 191 (2023), 104899 [36 pp.] ISSN: 0393-0440 Factor impacto JCR: 1.6 (2023) Categ. JCR: MATHEMATICS rank: 50 / 490 = 0.102 (2023) - Q1 - T1 Categ. JCR: PHYSICS, MATHEMATICAL rank: 25 / 60 = 0.417 (2023) - Q2 - T2 Factor impacto CITESCORE: 2.9 - Geometry and Topology (Q1) - Physics and Astronomy (all) (Q2) - Mathematical Physics (Q2)