Pasadena ad exchange firm OpenX has partnered with YP, operator of the Yellow Pages brand, to bring programmatic display advertising to the localized online and mobile directory service.

The multiyear agreement allows Atlanta’s YP to extend the audience and reach of its targeted display ads by using the real-time bidding platform offered by OpenX, which will give national advertisers access to YP’s display ad inventory. YP said more than 70 million visitors use its search tools each month in the U.S.

The partnership is effective immediately.

Jason Fairchild, OpenX’s chief revenue officer, said in an interview that roughly 25 percent of all online display ads are now sold through automated or “programmatic” technology, with that number expected to double in the coming years.

Fairchild said mobile ads are an increasing source of revenue for the company, which many industry watchers have said could be gearing up for a public offering in the near future.

“Its material and becoming more material,” Fairchild said of OpenX’s mobile ad revenue. While recent numbers were not available, OpenX said it had 2012 revenue of in excess of $150 million.