Two advertising companies formed a joint venture.

The new advertising firm, to be called Havas Dentsu Marsteller, will have more than $1 billion in billings and rank 17th in the world, officials said. Chief executive will be Tim Pollak, 41, who had been president of Young & Rubicam’s former joint venture with Tokyo-based Dentsu. The new firm, based in New York, is being created by a merger of Pollak’s company and a branch of Eurocom, one of the largest ad groups in Europe.