  We study the photoluminescence spectrum of a low density ($\nu <1$)
two-dimensional electron gas at high magnetic fields and low temperatures. We
find that the spectrum in the fractional quantum Hall regime can be understood
in terms of singlet and triplet charged-excitons. We show that these spectral
lines are sensitive probes for the electrons compressibility. We identify the
dark triplet charged-exciton and show that it is visible at the spectrum at
$T<2$ K. We find that its binding energy scales like $e^{2}/l $, where $l$ is
the magnetic length, and it crosses the singlet slightly above 15 T.
