AppNexus, the online advertising group in which WPP is major investor, is said to be considering a market flotation next year.The Sunday Times wrote that the company, which operates a software platform that connects internet sites and advertisers, is planning to list on the American stock exchange in a deal that would value the company at around £1.3bn.A source told the newspaper that AppNexus had not yet made a decision about whether to go ahead with the listing, but could wait until later in the year before doing so.The company has said that its financial position was "very strong" and that it was "able to control [its own destiny as to if and when we choose to pursue" a flotation.WPP is believed to hold around a 15% stake in the business.