Much of it has to do with Yahoo’s stake in Alibaba. On September 17, Alibaba raised $21.8 billion in the largest tech IPO ever recorded. After selling part of its stake in Alibaba, Yahoo’s share in the company is now worth around $44 billion, though Yahoo’s total market value is only $47 billion. Yahoo’s email service, website and other operations could be worth an additional $7 billion. That has led some shareholders to gripe that the stock price is undervalued. The investors cited by Reuters also said they believe a combined Yahoo and AOL could challenge Google and Facebook in the areas of video programming and the purchase of digital advertising. In its September letter, Starboard Value called out the area of digital advertising and said the combined companies could save up to $1 billion by cutting costs in the display business as well as in corporate overhead. Digital advertising — especially video and mobile ads — has become a key part of Mayer’s turnaround strategy. Yahoo on Tuesday announced it paid $640 million to acquire BrightRroll, a startup that helps customers buy and sell video ads on websites, mobile devices and connected TVs. The US market for Web video ads is expected to approach $6 billion by the end of 2014, said market research firm eMarketer.