  Photonic events with large missing energy have been observed in e+e-
collisions at centre-of-mass energies of 130, 136 and 183 GeV collected in 1997
using the OPAL detector at LEP. Results are presented for event topologies with
a single photon and missing transverse energy or with an acoplanar photon pair.
Cross-section measurements are performed within the kinematic acceptance of
each selection. These results are compared with the expectations from the
Standard Model process e+e- to nunubar + photon(s). No evidence is observed for
new physics contributions to these final states. Using the data at Ecm = 183
GeV, upper limits on sigma(e+e- to X.Y)*BR(X to Y gamma) and sigma(e+e- to
X.X)*BR(X to Y gamma)**2 are derived for the case of stable and invisible Y.
These limits apply to single and pair production of excited neutrinos (X = nu*,
Y = nu), to neutralino production (X = Chi_2^0, Y = Chi_1^0) and to
supersymmetric models in which X = chi_1^0 and Y is a light gravitino.
