The area around Gotha passed to Ernest 's eldest surviving son , Frederick of Saxe-Gotha-Altenburg , who also inherited Altenburg ( which Ernest had inherited through his wife Elisabeth Sophie ) and became Duke of Saxe-Gotha-Altenburg . The area around Gotha and also Altenburg passed to Frederick I , who retained the title of a Duke of Saxe-Gotha-Altenburg . For later history of the duchy , see Saxe-Gotha-Altenburg .