Siemowit of Dobrzyń, was a Polish prince member of the House of Piast, Duke of Brześć Kujawski during 1267–1288, Duke of Dobrzyń during 1288–1293, 1295–1303 and 1305–1312, during 1293–1295 in captivity in Lithuania, during 1303–1305 deposed, since 1306 hereditary vassal of the Kingdom of Poland. Wikipedia