Search for neutral heavy leptons produced in Z decays - Abreu, P; Adam, W; Adye, T; Ajinenko, I; Alekseev, GD; Alemany, R; Allport, PP; Almehed, S; Amaldi, U; Amato, S; Andreazza, A; Andrieux, ML; et al. (ZEITSCHRIFT FUR PHYSIK C-PARTICLES AND FIELDS, 1997)