We report on time-resolved photoluminescence studies of charged and neutral excitons in a modulation doped GaAs quantum well under resonant excitation and high magnetic field. The radiative lifetime of the charged exciton is rather short, 60 ps at zero held, and is found to increase by a factor of similar to 2 up to 7 T. The short lifetimes suggest that, under magnetic field, the exciton bound in the trion is delocalized. (C) 1998 Elsevier Science B.V. All rights reserved.