  Charged magneto-excitons X- in a dilute 2D electron gas in narrow and
symmetric quantum wells are studied using exact diagonalization techniques. An
excited triplet X- state with a binding energy of about 1 meV is found. This
state and the singlet are the two optically active states observed in
photoluminescence (PL). The interaction of X-'s with electrons is shown to have
short range, which effectively isolates bound X- states from a dilute e-h
plasma. This results in the insensitivity of PL to the filling factor nu. For
the "dark" triplet X- ground state, the oscillator strength decreases
exponentially as a function of 1/nu which explains why it is not seen in PL.
