Low-energy flavour probes of light vector bosons

. - In this work, we construct the chiral Lagrangian for a light spin-1 boson X possessing both vectorial and axial couplings to the light Standard Model quarks u, d, s. We then use it in order to describe the tree-level, model-independent contributions to the Delta S = 1 transition K +/- -> pi +/- X, which is induced by Standard Model charged currents and is possibly enhanced by the emission of a longitudinally polarized X boson. Such a flavour observable is then shown to set the best modelindependent bounds on the diagonal axial couplings of X to light quarks in the mass range allowed by the decay kinematics, improving the currently available constraints from beam-dump experiments and collider searches.