  If the gravitino G is very light and all the other supersymmetric particles
are above threshold, supersymmetry may still be found at colliders, by looking
at processes with only gravitinos and ordinary particles in the final state. We
compute here the cross-section for the process e^+ e^- ==> G G gamma, whose
final state can give rise to a distinctive photon plus missing energy signal at
present and future e^+ e^- colliders. We describe how the present LEP data can
be used to establish a lower bound on the gravitino mass of order 10^{-5} eV.
We conclude with a critical discussion of our results, comparing them with
related ones and outlining possible generalizations.
