  The production of single- and multi-photon events has been studied in the
reaction e+e- -> gamma (gamma) + invisible particles. The data collected with
the DELPHI detector during the years 1999 and 2000 at centre-of-mass energies
between 191 GeV and 209 GeV was combined with earlier data to search for
phenomena beyond the Standard Model. The measured number of light neutrino
families was consistent with three and the absence of an excess of events
beyond that predicted by the Standard Model processes was used to set limits on
new physics. Both model-independent searches and searches for new processes
predicted by supersymmetric and extra-dimensional models have been made. Limits
on new non-standard model interactions between neutrinos and electrons were
also determined.
