[Tokyo] Dentsu and Asatsu-DK, Japan's No. 1 and No. 3 ad agencies, will start a joint venture in October to develop next-generation advertising in Japan's changing media market. The unnamed company will develop content and "cross-media creative" that takes better advantage of digital TV, the Internet, mobile phones and other media. The deal is unusual because Dentsu and Asatsu-DK are direct competitors in Japan, the world's second-largest ad market. But both agencies have a common rival in Hakuhodo. Last year the No. 2 agency merged its business with Daiko Advertising and Yomiko Advertising into a joint holding company, Hakuhodo DY Holdings, to better compete against Dentsu, although they continue to operate as separate agencies.