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Fans of the The Big Bang Theory will instantly recognise the song Soft Kitty, which main character Sheldon Cooper uses to calm down whenever he feels anxious.
The nursery rhyme about a little kitten falling asleep has become synonymous with the CBS show, with a Big Bang Theory producer credited for writing the lyrics.
But the long-running comedy has been rocked by accusations it stole the song from a New Hampshire poet and teacher, who wrote and published the poem in the 1930s.
The daughters of school teacher Edith Newlin have filed a lawsuit against Warner Brothers and CBS and are suing them for copyright infringement, the New York Times reports.
Soft Kitty has been used on Big Bang Theory merchandise items, including air fresheners, fridge magnets, mouse pads and t-shirts.
"The Soft Kitty lyrics are among the best-known and most popular aspects of The Big Bang Theory," the lawsuit claims. "They have become a signature and emblematic feature of the show and a central part of the show's promotion."
According to the lawsuit, the Soft Kitty song was first published in 1937 in the book Songs for the Nursery School with the lyrics:
Soft kitty, warm kitty
Little ball of fur
Happy kitty, sleepy kitty
Purr, purr, purr.
It was copyrighted in 1937 and again in 1964.
Ellen Newlin Chase and Margaret Chase Perry said the media companies never requested permission to use the song, and "willfully infringed" copyright, claiming the song as their own.
They discovered the song had been used last year when one of the daughters was researching Newlin's life for an article she was writing.
In credits for The Big Bang Theory, Bill Prady, a producer at Chuck Lorre Productions, the show's creator, was attributed as the writer of Soft Kitty.
According to the document, the song has been used at least eight times on the show, which has the highest paid television actors in the world, from 2008 to 2015.
Warner Brothers reportedly secured rights to use the poem in 2007 after approaching the publisher of Songs for the Nursery School, Willis Music.
But Newlin remained the owner of the song's copyright and her daughters were never approached.
Warner Brothers are refusing to comment on the case.
That the song is not original will be of no surprise to Australian viewers who have been hearing the tune for decades.
Children's performer Patsy Biscoe released a version of Warm Kitty on her album, 50 Favourite Nursery Rhymes, which went platinum in the 1970s. It has also been performed repeatedly on ABC's Play School, with a recording released in 1993.
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