  Photonic events with large missing energy have been observed in e+e-
collisions at centre-of-mass energies of 130, 136, 161 and 172 GeV using the
OPAL detector at LEP. Results are presented based on search topologies designed
to select events with a single photon and missing transverse energy or events
with a pair of acoplanar photons. In both search topologies, cross-section
measurements are performed within the kinematic acceptance of the selection.
These results are compared with the expectations from the Standard Model
processes e+e- -> nu nu(bar) gamma (gamma) (single-photon) and e+e- -> \nu
\nu(bar) gamma gamma (gamma) (acoplanar-photons). No evidence is observed for
new physics contributions to these final states. Upper limits on sigma(e+e- ->
XY)*Br(X -> Y gamma) and sigma(e+e- -> XX)*Br**2(X -> Y gamma) 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^0_2, Y = \chi^0_1), and to supersymmetric models in which X =
\chi^0_1 and Y = \tilde{G} is a light gravitino. For the latter scenario, the
results of the acoplanar-photons search are used to provide model-dependent
lower limits on the mass of the lightest neutralino.
