  The production of single photons has been studied in the reaction e+e- ->
gamma + invisible particles at centre-of-mass energies of 183 GeV and 189 GeV.
A previously published analysis of events with multi-photon final states
accompanied by missing energy has been updated with 189 GeV data. The data were
collected with the DELPHI detector and correspond to integrated luminosities of
about 51 pb^{-1} and 158 pb^{-1} at the two energies. The number of light
neutrino families is measured to be 2.84 +/- 0.15(stat) +/- 0.14(syst). The
absence of an excess of events beyond that expected from Standard Model
processes is used to set limits on new physics as described by supersymmetric
and composite models. A limit on the gravitational scale is also determined.

High Energy Physics - Experiment