his maternal great-grandfather was philip, prince of eulenburg and hertefeld (1847–1921), a friend of wilhelm ii, whose youngest child viktoria ada astrid agnes gräfin zu eulenburg (1886–1967) married 1909 (div. 1921) professor otto ludwig haas-heye (1879–1959), and had issue, including two daughters. gustaf descends through both his mother and father from medieval scandinavian nobility and rulers.