  In photoluminescence spectra of symmetric [111] grown GaAs/AlGaAs quantum
dots in longitudinal magnetic fields applied along the growth axis we observe
in addition to the expected bright states also nominally dark transitions for
both charged and neutral excitons. We uncover a strongly non-monotonous, sign
changing field dependence of the bright neutral exciton splitting resulting
from the interplay between exchange and Zeeman effects. Our theory shows
quantitatively that these surprising experimental results are due to
magnetic-field-induced \pm 3/2 heavy-hole mixing, an inherent property of
systems with C_3v point-group symmetry.
