Piper, a pop singer and actress best known for her roles in "Doctor Who" and "Secret Diary of a Call Girl," previously played the character of Sally Lockhart in "The Ruby in the Smoke."

"Shadow," a co-production of the BBC and WGBH Boston, is to air Sept. 28 as part of PBS' "Masterpiece Mystery!" series, while "Ruby" is scheduled to be re-run Sept. 21. Both are based on the novels of Philip Pullman.

"Masterpiece" has been presented on PBS by WGBH since 1971.