  In the next-to-minimal supersymmetric standard model (NMSSM), the unique
$\lambda S H_u H_d$ in the superpotential gives rise to a coupling involving
the lighter pseudoscalar Higgs boson and a pair of charged or neutral
Higgsinos, even in the limit of zero mixing between the two pseudoscalar Higgs
bosons. We study the associated production of a very light pseudoscalar Higgs
boson with a pair of charginos. The novel signature involves a pair of charged
leptons from chargino decays and a pair of photons from the pseudoscalar Higgs
boson decay, plus large missing energy at the LHC and ILC. The signal may help
us to distinguish the NMSSM from MSSM, provided that the experiment can resolve
the two photons from the decay of the pseudoscalar Higgs boson.
