The abcgallery has this about her, "Josephina Fridrix (1780s-1824) was a mistress of Grand Duke Constantin Pavlovich (1779-1831), son of the Emperor Paul I. She had a son by him, Pavel Konstantinovich Alexandrov (1808-1857). In 1816, she received the Russian name of Uliana Mikhailovna Alexandrova. In 1820 she married A. S. Weis."
Her Russian language Wikipedia article is here.