The first "Jo, Zette & Jocko" adventure was titled "The Secret Ray", and began serialisation in "Cœurs Vaillants" on 19 January 1936. It would continue to appear in the newspaper in installments until June 1937, throughout being printed in red and black. Several months later it also began to appear in the pages of "Le Petit Vingtième". For New Year 1938, Hergé designed a special cover for "Le Petit Vingtième" in which the characters of "Jo, Zette and Jocko" were featured alongside those from "The Adventures of Tintin" and "Quick & Flupke".