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This image released by CBS shows Mayim Bialik, left, portrays Amy, and Jim Parsons portrays Sheldon in a scene from "The Big Bang Theory." On Thursday’s episode (8 p.m. EDT, CBS), Sheldon and Amy have sex for the first time, more than six TV seasons after meeting through an online dating site. (Michael Yarish/CBS via AP) Image Credit: AP

The heirs to a New Hampshire teacher who wrote a poem about a “soft kitty” say TV’s The Big Bang Theory is violating their copyrights.

Edith Newlin’s daughters sued CBS and other media-related companies Monday over the copyright to a song the lawsuit says has repeatedly been used on the sitcom.

According to the lawsuit, The Big Bang Theory 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 involving that phrase.

Messages seeking comment from CBS and other companies sued by Ellen Newlin Chase and Margaret Chase Perry were not immediately returned. Edith Newlin died in 2004.

The lawsuit seeks unspecified damages from the show’s producers and distributors.