  A variational calculation of the lower singlet and triplet states of
positively charged excitons (trions) confined to a single quantum well and in
the presence of a perpendicular magnetic field is presented. We study the
dependence of the energy levels and of the binding energy on the well width and
on the magnetic field strength. Our results are compared with the available
experimental data and show a good qualitative and quantitative agreement. A
singlet-triplet crossing is found which for a 200 \AA wide GaAs is predicted to
occur for B = 15 T.
