Celebrities that were big in the 1930s appeared in Betty Boop cartoons and would often perform their hit songs which were popular in the 1920s-1930s.
Maurice Chevalier had a small cameo in Stopping the Show, where he tells Betty Boop to impersonate him and right now. Maurice was a fan of the series, his hit song "Hello Beautiful" was featured in My Wife's Gone to the Country and Bimbo's Express in 1931.
Irene Bordoni performs the title song in Just a Gigolo. Betty introduces her by saying "Let's give little Irene a great big hand!"