Question: What country of origin does Brita Horn and Charles XIII of Sweden have in common?

Context articles: Swedish Parliament Act of 1810
Hedvig Elisabeth Charlotte of Holstein-Gottorp
King Gustav III of Sweden and his Brothers
Convention of Moss
Mariana Koskull
House of Bernadotte
Gustav IV Adolf of Sweden
Brita Horn
Charles XIII of Sweden
Order of Charles XIII

The 1810 Riksdag Act was passed by Parliament on 10 February 1810 during the reign of Charles XIII.
 It replaced the order of the estates meetings in 1617 reintroduced by Gustav III via the Union and Security Act instead of the Riksdag Act 1723.
 The 1810 parliamentary regime was adopted after Sweden lost Finland and Gustav IV Adolf was pushed aside in a coup.
 Charles XIII was subsequently appointed regent and then king, and after that a new form of government, the Government of 1809, was adopted.
 A notice of parliament which adopted the parliamentary order was issued in March 1809.
Hedwig Elisabeth Charlotte of Holstein-Gottorp (Swedish: "Hedvig Elisabeth Charlotta" ; 22 March 1759 – 20 June 1818) was Queen of Sweden and Norway as the wife of King Charles XIII and II.
 She was also a famed diarist, memoirist and wit.
 She is generally known in Sweden by her full pen name in Swedish (above), though her official name as queen was Charlotte.
King Gustav III of Sweden and his Brothers is an oil painting by the Swedish portrait painter Alexander Roslin showing Gustav with his two brothers, Prince Frederick Adolf and Prince Charles, later Charles XIII of Sweden.
 Frederick is standing, Gustav is sitting to the left, and Charles is to the right.
 The painting is in the collection of the Swedish National Museum.
The Convention of Moss was a cease fire agreement, signed on 14 August 1814 between the Swedish King and the Norwegian government.
 It followed the Swedish-Norwegian War due to Norway's claim to sovereignty.
 It also became the "de facto" peace agreement and formed the basis for the personal union between Sweden and Norway that was established when the Norwegian Storting (Parliament) elected Charles XIII of Sweden as king of Norway on 4 November 1814.
 The Union lasted until Norway declared its dissolution in 1905.
Henrietta Mariana "Marianne" Charlotta Koskull (1785–1841) was a Swedish noble and lady-in-waiting, known as the royal mistress of King Charles XIII of Sweden and King Charles XIV John of Sweden.
The House of Bernadotte ( ; ] ), the current royal house of Sweden, has reigned since 1818.
 Between 1818 and 1905, it was also the royal house of Norway.
 Its founder, Charles XIV John of Sweden (who was born Jean Bernadotte), was adopted by Charles XIII of Sweden, whose House of Holstein-Gottorp was becoming extinct.
Gustav IV Adolf or "Gustav IV Adolph" (1 November 1778 – 7 February 1837) was King of Sweden from 1792 until his abdication in 1809.
 He was the son of Gustav III of Sweden and his queen consort Sophia Magdalena, eldest daughter of Frederick V of Denmark and his first wife Louise of Great Britain.
 He was the last Swedish ruler of Finland, the occupation of which by Russian Emperor Alexander I in 1808-09 was the immediate cause of his violent downfall.
 After an army revolt, the king was seized by officers and forced to relinquish the throne on behalf of his family on March 29, the anniversary of his father's death (due to gunshot wound, in 1792).
 The Instrument of Government subsequently written was adopted on June 6, the current National Day of Sweden, and was in effect until replaced in 1974.
 The crown (now with strictly limited powers) passed to his childless uncle, Charles XIII, whose want of heirs set into motion an intense quest for a successor who was found the following year in the person of Jean-Baptiste Bernadotte, who assumed the throne in 1818, commencing the present House of Bernadotte.
Brita Margaretha Horn (1745 – 13 March 1791), was a Swedish countess and courtier.
 She is known for being the love interest of Charles XIII of Sweden from 1765 to 1771, during which Charles' wish to marry her was given political significance and used by the Caps (party) and Hats (party) during the age of liberty.
 She is also known for her correspondence with her spouse from 1775 to 1791, which has been partially published.
Charles XIII & II also Carl, Swedish: "Karl XIII" (7 October 1748 – 5 February 1818), was King of Sweden (as "Charles XIII") from 1809 and King of Norway (as "Charles II") from 1814 until his death.
 He was the second son of King Adolf Frederick of Sweden and Louisa Ulrika of Prussia, sister of Frederick II of Prussia.
The Royal Order of Charles XIII (Swedish: "Kungliga Carl XIII:s orden" ) is a Swedish order of merit, founded by King Charles XIII in 1811.