  A selection of di-lepton events with significant missing transverse momentum
has been performed using a total data sample of 237.4 pb-1 at e+e-
centre-of-mass energies of 183 GeV and 189 GeV. The observed numbers of events
- 78 at 183 GeV and 301 at 189 GeV - are consistent with the numbers expected
from Standard Model processes, which arise predominantly from W+W- production
with each W decaying leptonically. This topology is also an experimental
signature for the pair production of new particles that decay to a charged
lepton accompanied by one or more invisible particles. Discrimination
techniques are described that optimise the sensitivity to particular new
physics channels. No evidence for new phenomena is apparent and model
independent limits are presented on the production cross-section times
branching ratio squared for sleptons and for leptonically decaying charginos
and charged Higgs. Assuming a 100 % branching ratio for the decay of a slepton
to a lepton and the lightest neutralino, we exclude at 95 % CL: right-handed
smuons with masses below 82.3 GeV for a smuon-neutralino mass difference > 3
Gev and right-handed staus with masses below 81.0 GeV for a stau-neutralino
mass difference > 8 GeV. Right-handed selectrons are excluded at 95% CL for
masses below 87.1 GeV for a selectron-neutralino mass diffeerence > 5GeV,
within the framework of the Minimal Supersymmetric Standard Model assuming mu <
-100 GeV and tan{beta} = 1.5. Charged Higgs bosons are excluded at 95% CL for
masses below 82.8GeV, assuming a 100% branching ratio for their decay to
tau+neutrino.
