NEW YORK - Chemical Banking Corp. and shell Oil Co. announced their joint credit card on Monday, vowing to make it the largest cobranding effort yet.

Revealing the plan several months after its development had become an open secret, Chemical said it will initially market the Shell MasterCard to nine million people, including a third of the 12 million who have Shell's in-house card.

Chemical expects the "vast majority" of Shell cardholders to convert, said Charles R. Walsh, executive vice president in charge of the bank's retail card services.

Industry experts say Chemical could double its base of 4.5 million accounts, raising its industry rank by three places, to sixth.

It remains to be seen if the Chemical-Shell card - the largest cobranded product in the petroleum industry - can challenge the AT & T Universal card (about 12 million accounts) or Household Bank's General Motors MasterCard (5 million).

Shell represents one more victory for MasterCard, which is the brand of the lion's share of cobranded credit cards. Chemical said it has no plans to introduce a Shell Visa version.

A number of major oil companies have converted parts of their in-house credit card businesses to bank cards, offering a variety of rebates.

But oil companies generally face an uphill fight getting their customers to revolve their monthly charges.

The Shell MasterCard lets cardholders pay for gasoline purchases in full without incurring interest, but gives them the option of revolving other charges.

10% Annual Saving Claimed

Chemical and Shell say their card can save customers 10% in annual gas costs.

During the first year, cardholders can get back 2% on general purchases and a 1% bonus for a 3% total rebate on Shell gas purchases.

After the first $70 in rebates, cardholders earn 1% for the remainder of the first year.

Shell customers who convert to the MasterCard, and others who sign up by April 30, will not have to pay the $20 annual fee.

New customers can transfer balances at the prime rate plus 9%, or 15%, or prime plus 10.4%, or 16.4% rate, depending on the qualifications they meet.

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