The heirs to a New Hampshire teacher who wrote a poem about a "soft kitty" eight decades ago said in a lawsuit Monday that TV's The Big Bang Theory is violating their copyrights.

Edith Newlin's daughters sued CBS and other media-related companies over the copyright to a song the lawsuit says has repeatedly been used on the hit sitcom. According to the lawsuit, The Big Bang Theory, one of the highest-rated shows on television, used lyrics written by Newlin in the 1930s without buying the rights. The lyrics begin: "Soft kitty, warm kitty."

The Big Bang Theory characters have periodically sung a lullaby called "Soft Kitty" involving that phrase, often to comfort theoretical physicist Sheldon Cooper, played by Jim Parsons. The song has been sung eight times over the years, beginning with the "The Pancake Batter Anomaly" episode in 2008. Watch a compilation:

A spokeswoman for CBS declined to comment. Messages seeking comment from other companies sued by Ellen Newlin Chase and Margaret Chase Perry were not immediately returned Monday.

Edith Newlin died in 2004. She had worked as a nursery school teacher in Alstead, New Hampshire, for about 35 years. Her daughters still live in the small town.

The daughters' lawsuit, seeking unspecified damages from the show's producers and distributors, says the "soft kitty" lyrics have been used in their entirety on at least eight episodes of the show since March 2008. It says the lyrics also have been used in merchandising, including being displayed in their entirely on T-shirts, sweatshirts, hoodies, pajamas, mouse pads, mobile phone covers, wallets, air fresheners, refrigerator magnets, singing plush toys and other products.

And it says the defendants not only have failed to credit Newlin but have sometimes made it appear as if the lyrics were written by one of the show's producers.

"The Soft Kitty Lyrics are among the best-known and most popular aspects of The Big Bang Theory," the lawsuit says. "They have become a signature and emblematic feature of the show and a central part of the show's promotion."

At three or more Comic-Con conventions since 2010, producers and actors from the sitcom led large audiences in singing the lyrics, the lawsuit says.

According to the lawsuit, Ellen Newlin Chase learned of the show's use of the lyrics in August 2014, when she was researching her mother's history for an article she was writing and came across a blog post discussing the use of the lyrics on the show.

The lawsuit says Warner Bros. Entertainment and the show's other producers decided in 2007 that they wanted to use the lyrics and sought permission from Willis Music Co., a Florence, Kentucky-based company that had published them in a book, "Songs for the Nursery School."

The lawsuit says Willis Music authorized use of the lyrics without consulting or getting permission from Newlin's heirs even though the book makes clear on its acknowledgement page and where the lyrics appear that Newlin was the author of and owned the copyright to the lyrics.

Messages for comment left with Warner Bros. Entertainment Inc., based in Burbank, California, and Willis Music were not immediately returned.