  Single and multi-photon events with missing energy are analysed using data
collected with the L3 detector at LEP at a centre-of-mass energy of 189 GeV,
for a total of 176 pb^{-1} of integrated luminosity. The cross section of the
process e+e- -> nu nu gamma (gamma) is measured and the number of light
neutrino flavours is determined to be N_\nu = 3.011 +/- 0.077 including lower
energy data. Upper limits on cross sections of supersymmetric processes are set
and interpretations in supersymmetric models provide improved limits on the
masses of the lightest neutralino and the gravitino. Graviton-photon production
in low scale gravity models with extra dimensions is searched for and limits on
the energy scale of the model are set exceeding 1 TeV for two extra dimensions.
