Peter III of Russia
Empress Elizabeth arranged for Peter to marry his second cousin, Sophia Augusta Frederica (later Catherine the Great), daughter of Christian August, Prince of Anhalt-Zerbst and Johanna Elisabeth of Holstein-Gottorp. The young princess formally converted to Russian Orthodoxy and took the name Ekaterina Alexeievna (i.e., Catherine). They married on 21 August 1745. The marriage was not a happy one but produced one son, the future Emperor Paul, and one daughter, Anna Petrovna (20 December 1757 – 19 March 1759). Catherine later claimed that Paul was not fathered by Peter: that, in fact, they had never consummated the marriage. During the sixteen years of their residence in Oranienbaum, Catherine took numerous lovers, while her husband did the same in the beginning.
Tsesarevich
In 1721 Peter the Great discontinued use of "tsar" as his main title, and adopted that of "imperator" (emperor), whereupon the title of tsarevich (and "tsarevna", retained for life by Ivan V's daughters) fell into desuetude. The Emperor's daughters were henceforth referred to as ""tsesarevna"" (Peter had no living son by this time). In 1762, upon succeeding to the imperial throne, Peter III accorded his only son Paul Petrovich (by the future Catherine the Great) the novel title of "tsesarevich", he being the first of nine Romanov heirs who would bear it. However, at the time the title was conferred, Paul was recognized as Peter's legal son, but not as his legal heir. Nor would he be officially recognized as such by his mother after her usurpation of the throne.