Search for a nearly degenerate lepton doublet (L-,L0)

We have searched for a heavy charged lepton with an associated neutrino of nearly the same mass in {ital e}{sup +}{ital e}{sup {minus}} annihilation data taken with the Mark II detector at a center-of-mass energy of 29 GeV. In order to suppress contamination from conventional two-photon reactions, this analysis uses a novel, radiative-tagging technique. Requiring the presence of an isolated, energetic photon allows a search for lepton doublets with mass splittings smaller than that previously accessible to experiment. No evidence for such a new lepton doublet has been found, enabling limits to be placed on allowed mass combinations. Mass splittings as low as 250--400 MeV/{ital c}{sup 2} are excluded for charged-lepton masses between 500 MeV/{ital c}{sup 2} and 10 GeV/{ital c}{sup 2}.