  A search for events with an imbalance in transverse momentum and with
isolated high energy leptons has been carried out at the positron-proton
collider HERA. One event with an e^- and five events with a mu^+- are found
together with evidence for undetected particles carrying transverse momentum.
Within the Standard Model the dominant origin of events with this kind of
topology is the production of W bosons with subsequent leptonic decay. Three of
the six events are within measurement errors found in a region of phase space
likely to be populated by this process, while the remaining events show
kinematic properties which are atypical of all Standard Model processes
considered.
