LOS ANGELES — Benedict Cumberbatch is a star used to working with other stars, but even he was impressed by the cast of a new project.

The British actor of “Sherlock” and “Star Trek” fame appears with a pair of “Sesame Street” Muppets in a video posted Tuesday on PBS’ YouTube channel.

With the help of Count von Count (Matt Vogel, who replaced the late Jerry Nelson) and Murray Monster (Joseph Mazzarino), Cumberbatch explains how to count up four apples and three oranges, and which is the greater number.

He also, patiently and repeatedly, tries to get a skeptical Murray to believe that he is not actually Sherlock Holmes but just plays the detective on TV.

Cumberbatch taped the spot in January, when he joined actress Amanda Abbington to promote the new “Sherlock” season to a TV critics meeting in Pasadena, Calif. His puppet encounter proved to be something to brag about.

“I just met with the Muppets,” Cumberbatch gleefully told Abbington, who plays the wife of Dr. Watson (Martin Freeman, her real-life partner).

“It was so much fun,” he said, adding, “They are real.”