Critical or tricritical SAd-SA 2 point in a (temperature-molecular length) phase diagram ?

From microscopic observations, DSC experiments and X-ray data the Smectic Ad-Smectic A 2 transition is studied in seven compounds of the 4-n alkoxy phenyl-4' cyanobenzoyloxy benzoate series (DBnO). In particular the excess enthalpy associated with this transition is found to depend on the width of the SAd range. These results suggest either tricritical or critical behaviour in a (T, molecular length) phase diagram.