  Absorption spectra fine structure of $KDy(MoO_4)_2$ in the region of
cooperative Jahn-Teller type ordering was studied. Temperature anomalies in the
spectra occurring at phase transformation correlate with the ultrasound
peculiarities observed earlier. Based on the symmetry approaching, possible
activity of the irreducible representations of the rhombic $D_{2h}$ point group
was discussed, which lead to the incommensurate phase at cooperative ordering.
It was supposed, that coupled $A_u$-type phonon mode may lead to the
incommensurate phase existence, which is possible at least in the temperature
region 17-12 K.
