U.S. authorities are investigating Steven A. Cohen's SAC Capital Advisors hedge fund for alleged insider trading in the shares of the popular diet company Weight Watchers International Inc, according to people familiar with the matter.

The sources said on Friday the investigation could implicate the billionaire hedge fund manager and focuses on trading in Weight Watchers shares in the first half of 2011, when SAC Capital had taken a sizeable position in the stock.

An SAC Capital spokesman said the firm was not aware of any investigation involving Weight Watchers. A spokeswoman for Manhattan U.S. Attorney Preet Bharara declined to comment. A spokesperson for Weight Watchers did not immediately respond to a request for comment.

Regulatory filings show that Cohen's $14 billion fund briefly held 2.1 million shares in Weight Watchers during the period under scrutiny by authorities - at which time the diet company's stock price roughly doubled.

The inquiry is in its early stages and the people familiar with the matter, who requested anonymity, said it is not clear whether anything improper was done either by SAC Capital or Cohen himself. The trading in Weight Watchers would be permissible as long as it was based on fundamental research or derived from individuals who did not have access to non-public corporate information.

(Reporting by Matthew Goldstein, Jennifer Ablan and Emily Flitter; Editing by Tiffany Wu, Bernard Orr)