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)over bar>gamma(gamma) is measured and the n
umber of light neutrino flavours is determined to be N-nu= 3.011 +/- 0.077
including lower energy data. Upper limits on cross sections of supersymmetr
ic processes are set and interpretations in supersymmetric models provide i
mproved limits on the masses of the lightest neutralino and the gravitino.
Graviton-photon production in low scale gravity models with extra dimension
s is searched for and limits on the energy scale of the model are set excee
ding 1 TeV for two extra dimensions. (C) 1999 Elsevier Science B.V. All rig
hts reserved.