[*Or three if you count K9.]
Manning accompanied Third Doctor Jon Pertwee between 1971 and 1973.
She and Smith will appear in a two-episode run of the spin-off, which is aimed at a younger audience than the parent show.
Full BBC press release here.
Sarah Jane Smith (travelled in the TARDIS between 1973 and 1976, taking over as The Doctor’s companion after Manning left. She appeared alongside both Pertwee and fourth Doctor Tom Baker.
She was reunited with The Doctor (in the guise of David Tennant) during the second series of the revived Doctor Who in 2006. The character, arguably the show’s all-time favourite companion, was then given her own spin-off show, The Sarah Jane Adventures, which also included occasional appearances by the Doctor’s robot dog K-9 (who was reintroduced full-time in season three).
Incidentally, Katy Manning famously caused a bit of a stir when, shortly after leaving Doctor Who, she posed naked with a Dalek for a saucy nude magazine photoshoot.