Who married Beatrice, Countess of Montfort?

Beatrice de Montfort, Countess of Montfort-l'Amaury (December 1249 – 9 March 1312) was a ruling sovereign countess of Montfort from 1249 until 1312. She was also countess of Dreux by marriage to Robert IV, Count of Dreux. She was the ancestor of the Dukes of Brittany from the House of Montfort-Dreux which derived its name from her title.

Robert IV of Dreux (1241–1282), Count of Dreux, Braine and Montfort-l'Amaury, was the son of John I of Dreux and Marie of Bourbon.

Robert fought with Philip III of France in 1272 in his expedition to the Languedoc and was present at the capture of Foix.

In 1260 he married Beatrice de Montfort, Countess of Montfort-l'Amaury daughter of Jean I de Montfort and Jeanne, Dame de Chateaudun, and granddaughter of Amaury de Montfort. Their son John succeeded his father while their eldest daughter Yolande (1263–1322) married Alexander III, King of Scots, and later Arthur II, Duke of Brittany.