  Hidden sector particles with sub-GeV masses like hidden U(1) gauge bosons,
the NMSSM CP-odd Higgs, and other axion-like particles are experimentally
little constrained as they interact only very weakly with the visible sector.
For masses below the muon threshold, we present constraints from meson decays,
g - 2 as well as beam-dump and reactor experiments. The NMSSM CP-odd Higgs and
generally any pseudoscalar is required to be heavier than 210 MeV or couple to
fermions much weaker than the SM Higgs. Hidden photons are less constrained and
can be searched for at future fixed-target experiments, e.g. HIPS at DESY.
