  In the present paper the coefficients of piezooptic (PO), electrooptic (EO)
and combined piezoelectrooptic (PEO) effects for LiNbO3 and LiTaO3 crystals are
reported. It is shown that the combined PEO effect is interchangeable, i.e. its
coefficients determined independ-ently from the EO and PO experiments are the
same.
