 Francis Stephen remained at the imperial court until 1729 , when he ascended the throne of Lorraine , but was not formally promised Maria Theresa 's hand until 31 January 1736 , during the War of the Polish Succession . Louis XV of France demanded that Maria Theresa 's fiancé surrender his ancestral Duchy of Lorraine to accommodate his father @-@ in @-@ law , Stanisław I , who had been deposed as King of Poland . Francis Stephen was to receive the Grand Duchy of Tuscany upon the death of childless Grand Duke Gian Gastone de ' Medici . The couple were married on 12 February 1736 . 
