Big Bang Theory season 9 episode 23
Kaley Cuoco as Penny in The Big Bang TheoryCBS

The Big Bang Theory added Katey Sagal to its cast for season 10 as Penny's mother Susan. Kaley Cuoco, who plays Penny in the CBS show, took to social media to share her excitement about the new cast update.

Sagal had played Cuoco's TV mother in 8 Simple Rules, which ran from 2002 to 2005 on ABC network. The Big Bang Theory actress shared a throwback image of herself with Sagal on Instagram and wrote, "Yes yes yes yes !!! Once my tv mom, always my tv mom love you@kateylous !!!!! #fullcircle@thebigbangtheory"

Other than Sagal, showrunner Steve Molaro revealed at Comic-Con that Jack McBrayer would join the show as Penny's brother Randall in season 10. According to CBS, Sagal's character is described as stressed out and neurotic from a life with her troubled son, while McBrayer will play a cheerful former drug dealer and ex-con.

The Big Bang Theory's executive producer Steve Molaro is also excited about the new cast. Molaro told ET Online, "There will be a little reunion there, which we're excited about. She's [ Sagal] going to be a little bit of a neurotic character because she's spent her life with her troubled son, Randall, who has been in and out of jail and has had some issues with dealing drugs. So we're going to start to dive into that."

Melissa Rauch, who plays Bernadette in the show, is also excited about McBrayer casting as Penny's brother. She said, "I'm very excited! I just found about this casting news the other day, and I was so excited. And Jack McBrayer playing Penny's brother is so exciting!"

The Big Bang Theory returns with its 10th season on 19 September at 8pm EST on CBS Network.