The phase diagram of a mixture of the discogens triphenylene hexa-n-heptanoate (HET7) and triphenylene hexa-(2-methyl-4-n-decyloxy)benzoate is presented. This mixture provides a more accessible nematic temperature range than was previously possible. The refractive indices of one mixture composition are measured and a rather small, negative birefringence found. The two-parameter Haller extrapolation method is used to calculate the molecular polarizability ratio Δα/α. This has a rather small magnitude compared with calamitic mesogens, although the individual polarizabilities are much larger, due to the greater molecular mass of these compounds. The calculated value of Δα/αis used to give the uniaxial orientational order parameterS, whose value is comparable to those of calamitic examples. Tabulated bond polarizability data are used to predict a value for Δα/α, which is found to be similar to the experimental result.