  It is shown that the time-reversal symmetry is broken during the phase
transition causing orbital ordering in a crystal with Jahn-Teller (J-T) ions.
This means that such phase transitions are magnetic ones. As an example we have
considered the spinel crystals containing $T_{2g}$ ions: $NiCr_2O_4$
($T_{orb}=310$ K, $T_c=70$ K) and $CuCr_2O_4$ ($T_{orb}=860$ K, $T_c=135$ K),
where the orbital ordering precedes the spin ordering, which occurs at lower
temperatures. The spin-ordering transition is naturally included into our
general group-theoretical scheme of the phase transition. Based on the symmetry
group of the parent phase, the symmetry groups for possible orbital- and
spin-ordered phases are found. We present arguments in favor of the idea that
the orbital ordering occurs due to the superexchange between J-T ions, and the
spin ordering is caused by the spin-orbital interaction. At the same time, the
displacements of atoms of the coordination polyhedron do not lead to the
removal of the orbital degeneracy, and they are merely an accompanying effect.
