Joan Brown

According to one account, Joan Brown was a companion to the Fourth Doctor.

She assisted him in his fight against the Zirconians. (COMIC: Doomcloud)

Behind the scenes
Joan Brown's sole appearance is in the Mighty Midget Doctor Who Comic reprint of Doomcloud, in which she replaced Sarah Jane Smith, while the Third Doctor was replaced by the Fourth Doctor. As TV Comic's depiction of Sarah was so unlike that of Elisabeth Sladen, no alteration to the artwork was necessary in this case. However, the Doctor does still refer to "Joan" as "Sarah" in one panel that was presumably overlooked in the updating process.