Paul I of Russia
Paul I (; "Pavel Petrovich") ( – ) reigned as Emperor of Russia between 1796 and 1801. Officially, he was the only son of Peter III and Catherine the Great, though Catherine hinted that he was fathered by her lover Sergei Saltykov, who also had Romanov blood, being a descendant of the first Romanov tsar's sister, Tatiana Feodorovna Romanova.
A Thief in the Night (Cornwell book)
Cornwell concluded that John Paul I died of a pulmonary embolism (which was supported by the fact that he had experienced a retinal embolism in 1976).