Viacom has joined Comcast and NBCUniversal as investors and contributors to Zeebox, a second-screen app for TV that made its U.S. debut last month.

The conglom will provide exclusive content and financial and promotional support to the venture, which was a breakthrough product in its home base in the U.K. Comcast and NBCU signed on last week. HBO and sister network Cinemax also struck a deal with Zeebox but their participation does not extend to investment. With another high-powered programmer on board, Zeebox is making impressive strides to take a leadership position in the fragmented marketplace for companion content experiences to TV shows on digital platforms, a second screen content companies and advertisers are hoping will reinforce the value of first-run fare and deepen viewer engagement.

Zeebox provides an assortment of extensions to a TV program, from relevant social-media chatter to e-commerce opportunities. The service is available either as an app on wireless devices or via browser.