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Duke of Aveiro (Portuguese: Duque de Aveiro) was a Portuguese title of nobility, granted in 1535 by King John III of Portugal to his 4th cousin, John of Lencastre, son of Infante George of Lencastre, a natural son of King John II of Portugal.
John of Lencastre was already Marquis of Torres Novas when the King granted him the new title of Duke of Aveiro. Later, their descendants strongly supported Philip II of Spain during the 1580 Portuguese succession crisis. Thus the Dukes became the second aristocratic House of Portugal, after the Braganzas.
Raimundo of Lencastre, 4th Duke of Aveiro maintained his House's traditional support for the Habsburg monarchy, even after the 1640 national revolution in Portugal. Therefore the Aveiro property was confiscated by the new Kings of the Braganza Dynasty, and granted in 1668 to his uncle, Peter of Lencastre, who already was Archbishop of Évora and general Inquisitor, becoming 5th Duke of Aveiro. He died in 1673 without issue.
The succession was given to his niece, Maria de Guadalupe of Lencastre, who was married to the Spanish Duke of Los Arcos. She returned to Portugal with her younger son, while her husband and her older son stayed in Spain. She regained the control of her House and estates and recognised the suzerainty of the House of Braganza.
Due to the alleged participation of the 8th Duke of Aveiro in the Távora affair (a conspiracy against the King), all the possessions of the Dukes of Aveiro were confiscated, their coat-of-arms destroyed from the public places, their houses demolished and their lands salted. Only the Palace of the Dukes of Aveiro in Azeitão, Portugal remains (albeit with the destroyed coat of arms on its facade) as a testament to the family's previous might and power.
The Aveiro palace in Lisbon, demolished and salted, gave place to a stone memorial in order to perpetuate the memory of the shame of the House of Aveiro. In it the following text can be read (in Portuguese): Aqui foram arrasadas e salgadas as casas de José Mascarenhas, exautorado das honras de Duque de Aveiro e outras condemnado por sentença proferida na Suprema Juncta de Inconfidencia em 12 de Janeiro de 1759. Justiçado como um dos chefes do barbaro e execrando desacato que na noite de 3 de Septembro de 1758 se havia commetido contra a real e sagrada pessoa de D.José I. Neste terreno infame se não poderá edificar em tempo algum. ("In this place were put to the ground and salted the houses of José Mascarenhas, stripped of the honours of Duque de Aveiro and others, convicted by sentence proclaimed in the Supreme Court of Inconfidences on the 12th of January 1759. Put to Justice as one of the leaders of the most barbarous and execrable upheaval that, on the night of the 3rd of September 1758, was committed against the most royal and sacred person of the Lord Joseph I. In this infamous land nothing may be built for all time.")
After 1759 the title became abeyant. In 1939, D. Duarte Nuno, Duke of Braganza, authorized the use of the personal title of Duke of Aveiro to D. Caetano Henriques Pereira Faria Saldanha de Lancastre, Count of Alcáçovas, and of Duke of Torres Novas to his eldest son. Neither are descended from the Dukes of Aveiro and did not use the prerogative. The only legitimate relatives of José de Mascarenhas da Silva e Lancastre, 8th Duke of Aveiro, are descended from his sister Francisca das Chagas Mascarenhas and her husband the 1st Marquis of Lavradio.