US media personality Oprah Winfrey is paying $43m (£28m) for a 10% slice of slimming firm Weight Watchers.
She will also get a seat on the board as part of the deal, which will grant the company the right to use her image and name on its goods and adverts.
Ms Winfrey is buying 6.4 million shares of Weight Watchers at $6.79 each and will have the option to buy another 5% of the company.
Shares soared more than 100% to $14 in New York following the announcement.
“We are expanding our purpose from focusing on weight loss alone to more broadly helping people lead a healthier, happier life,” said Weight Watchers chief Jim Chambers.
Ms Winfrey, 61, will take part in the Weight Watchers programme and candidly track her experiences for members. She has spoken publically about battles with her weight in the past.
“I believe in the program so much I decided to invest in the company and partner in its evolution,” she said.
Neil Saunders, managing director of retail research firm Conlumino, said: “The good thing about picking Oprah Winfrey is that although she is very much a really strong media personality, lots of people still see her as being a very ordinary person – a person that has struggled with things like weight loss.”
Weight Watchers is one of the world’s leading weight loss brands, but fewer people have been subscribing to classes, with rival mobile apps and activity monitors a contributing cause.
In the US, where the company has the majority of its subscribers, the number signing up has fallen by more than 20% in the past four years.
In April, the founder of Weight Watchers, Jean Nidetch, died aged 91.
In 1978, the company was sold to Heinz for $71m and she retired to south Florida.
Weight Watchers went public in 2001. In February the company’s stock fell to a record low after disappointing results.
The company is best known for its in-person weight loss coaching, where members meet to discuss their progress and difficulties.
In the past decade it has expanded its business to include meal plans, exercise trips and online coaching.
Weight Watchers sells branded ready meals and snacks and has licensing deals with low calorie food brands such as Yoplait yogurt.