When you signed up for your creditcard, the issuer probably made you abunch of promises, including a fixed interestrate. Guess what? That fixed rate isn'tcarved in stone. The credit card issuer canchange it by giving you as little as 15 days'notice. As the Federal Reserve bumpsinterest rates up, watch your mailbox fornotices that your credit card company isratcheting your interest rate up a notch.Some issuers, including MBNA Corp andWells Fargo, have already notified cardholdersof higher rates. Higher rates arebad news for the estimated 35 millionAmerican households who are either overtheir card limit or behind in payments onat least one card. The average US householdis carrying $6300 of credit card debt,according to government figures.