However, the Salic law excluded her, as a woman, from the succession to her father, while she was entitled to succeed her mother since it was derogated in the Duchy of Massa and Carrara by virtue of a 1529 decree of the Emperor Charles V. When it became obvious that the princely couple would not produce a large offspring, the reigning Duke, Francis III, set out to prevent Modena from suffering the same fate as Ferrara almost two centuries earlier, simply being reincorporated into the Empire as a vacant imperial fief.

Thus, in 1753, two simultaneous treaties (one public and one secret) were concluded between the House of Este and the House of Austria, by which the Archduke Leopold, Empress Maria Theresa's ninth-born child and third son, and Maria Beatrice were engaged, and the former was designated by Francis III as heir for the imperial investiture as Duke of Modena and Reggio in the event of extinction of the Este male line.

In January 1771 the Perpetual Diet of Regensburg ratified Ferdinand's future investiture and, in October, Maria Beatrice and he finally got married in Milan, thus giving rise to the new House of Austria-Este.

Ercole was compensated with the small principality of Breisgau in southwestern Germany, and when he died in 1803, it passed to his son-in-law, who in 1806 lost it to the enlarged and elevated Grand Duchy of Baden during the Napoleonic reorganization of the western territories of the defunct Holy Roman Empire.

When she died in 1829, she too was succeeded as ruler of Massa and Carrara by Francis IV, who in a few years completely assimilated her ancient Tuscan domains within the 'Este States' (Stati Estensi), as his Duchy was officially styled.

After the death of his mother Maria Beatrice of Savoy in 1840, he was considered the legitimate heir to the English and Scottish thrones by the Jacobites (with the regnal title King Francis I).

[1] In 1896 he became the heir presumptive of Austria-Hungary and, according to the terms of the secundogeniture, could not combine the Austria-Este inheritance with that of the main line of the House of Habsburg, i.e., the Austro-Hungarian Empire; but he was assassinated 28 June 1914 in Sarajevo before becoming emperor.