San Francisco ? Internet powerhouses Yahoo and eBay are joining forces in an alliance that further defines the battle lines in an online brawl with rivals Google, Microsoft and AOL.

Under the multiyear partnership announced Thursday, Yahoo and eBay will draw upon each other’s strengths in online advertising, payments and communications so they can connect with even more Web surfers than they already do.

Sunnyvale, Calif.-based Yahoo Inc. operates the world’s most trafficked Web site with 402 million unique users and ranks second in the lucrative search advertising market behind Google.

San Jose, Calif.-based eBay Inc. is the e-commerce leader, with nearly 200 million users of online auctions and another 73 million U.S. account holders of its payment service, PayPal.