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.
Russian interregnum of 1825
Alexander I of Russia, the elder of four sons of Paul I, had no male issue; his legitimate daughters died in infancy. According to the 1797 Pauline Laws his childless brother Constantine had been heir presumptive since Alexander's accession. The third brother, Nicholas, followed him in the order of succession. Constantine and his legitimate wife Juliane of Saxe-Coburg-Saalfeld separated in 1799. Juliane returned to Germany and resisted any proposals to restore the family. Constantine, engaged in the action of the Napoleonic Wars, did not attempt a formal divorce until meeting Joanna Grudzińska. Their love affair that began in 1815 compelled Constantine to divorce Juliane and marry Joanna.