Optically active compounds of the formula ##STR1## wherein R.sup.1 and R.sup.2 each are alkyl with at least 2 carbon atoms; A.sup.1, A.sup.2 and A.sup.3 each are unsubstituted, cyano-, halogen- or methyl-substituted 1,4-phenylene, optionally having one or two CH groups replaced by nitrogen, unsubstituted, cyano- or methyl-substituted trans-1,4-cyclohexylene, the cyclohexylene optionally having one or two CH.sub.2 groups replaced by oxygen; Z.sup.1 and Z.sup.2 each are a single covalent bond, --COO--, --OOC--, --CH.sub.2 CH.sub.2 --, --OCH.sub.2 --, --CH.sub.2 O--, --C.tbd.C--, --COS-- or --SOC--; m stands for the number 0 or 1; n is an integer of 0 to 4; X is oxygen or a methylene group; and each C* represents a chiral carbon atom, one of the chiral carbon atoms has the R-configuration while the other has the S-configuration.The compounds are used in liquid crystalline mixtures to compensate the temperature dependence of the threshold potential of liquid crystal cells.