Grand Duke Guillaume swore an oath to ascend to the throne of Luxembourg on Friday in a traditional ceremony attended by royals and European heads of state. AdvertisementFlanked by his wife, the new Grand Duchess Stéphanie, Grand Duke Guillaume, 43, swore the oath in Luxembourg’s parliament and took the throne. I will share their joys and sufferings,” Guillaume said, quoting a 1919 speech by one of his predecessors, the Grand Duchess Charlotte, in his first address to the nation. Guillaume, who becomes the new grand duke, and his wife, Stephanie. Some had their faces painted in the Luxembourg flag: red, white and light blue – the same colour of gown worn by the new Grand Duchess, Belgian-born Stéphanie.