NEW YORK, June 27 (Reuters) - Virgin Mobile USA Inc VM.N said on Friday it plans to buy Helio, the U.S. mobile arm of South Korea's SK Telecom Co Ltd (017670.KS: Quote, Profile, Research), for $39 million in equity.

Virgin Mobile, which is partly owned by Richard Branson's Virgin [VA.UL] and Sprint Nextel Corp (S.N: Quote, Profile, Research), serves more than 5 million customers. Its growth has been slowing amid growing competition from rivals and a weaker U.S. economy.

The deal is expected to close in the third quarter of 2008, and Virgin Mobile said the Virgin Group and SK Telecom will each invest $25 million of equity capital in the company.

SK Telecom, South Korea's top mobile operator, said in May it was in preliminary talks with Virgin Mobile about a deal involving Helio, a money-losing venture owned 69 percent by SK and 28 percent by EarthLink Inc (ELNK.O: Quote, Profile, Research). (Reporting by Ritsuko Ando; editing by Jeffrey Benkoe)