  We show that optical transitions of charged excitons in semiconductor
heterostructures are governed in magnetic fields by a novel exact selection
rule, a manifestation of magnetic translations. It is shown that the
spin-triplet ground state of the quasi-two-dimensional charged exciton X- --- a
bound state of two electrons and one hole --- is optically inactive in
photoluminescence at finite magnetic fields. Internal bound-to-bound X- triplet
transition has a specific spectral position, below the electron cyclotron
resonance, and is strictly prohibited in a translationally-invariant system.
These results allow one to discriminate between localized and free charged
excitons.
