Mark Warbington, my friend and a Doctor Who expert, sent me an email earlier today about the passing of Elisabeth Sladen, the actress who played one of the most well-known and loved Doctor Who companions Sarah Jane Smith. Sladen established the companion as a more important role in the Doctor Who universe than a didactic role. Her popularity also led to her reprising her role as Sarah Jane Smith beyond her first work in the Doctor Who universe as companion to the third and fourth Doctors (Jon Pertwee and Tom Baker, respectively) from 1973-1976. She played Sarah Jane Smith in subsequent Doctor Who television and radio dramas, and she starred in the spin-off series The Sarah Jane Adventures. Sladen will be missed.
The Wikipedia articles on Sladen and Sarah Jane provide a wealth of information on her life and work. Naomi Alderman has a nice reflection on Sladen in the Guardian here. Dan Martin contributed her obituary in the Guardian here.