Weight Watchers International Inc.’s stock got another boost from the falling weight of Oprah Winfrey, the company’s backer and most famous customer. The shares climbed as much as 19 percent after Winfrey announced that she’d lost 40 pounds using the program. Winfrey owned about 6.4 million shares, or about 10%, of Weight Watchers as of Dec. 15, which makes her the third-largest shareholder. That means she could be making $8.1 million on her Weight Watchers stock Thursday.