The Bobbsey Twins are the principal characters of what was, for 75 years, the Stratemeyer Syndicate's longest-running series of American children's novels, written under the pseudonym Laura Lee Hope. The first of 72 books were published in 1904, the last in 1979, with a separate series of 30 books published from 1987 through 1992. The books related the adventures of the children of the upper-middle-class Bobbsey family, which included two sets of fraternal twins: Nan and Bert, who were twelve years old, and Flossie and Freddie, who were six.