A common type of variable-rate financing has suddenly increased the risk of bankruptcy for an unknown number of colleges, said two experts at credit-rating agencies. The threat affects mostly less-wealthy colleges that have used variable-rate bonds either to get a better price when borrowing or because they lacked bond ratings and had little other choice, said the analysts, from Moody's Investors Service and Standard & Poor's. The analysts, Mary Peloquin-Dodd, of Standard & Poor's, and Roger D. Goodman, of Moody's, declined to predict whether and how many colleges might actually face bankruptcy, saying much still depends on the speed of an economic recovery and on what else the federal government does to help a recovery along. They agreed, however, that the use of variable-rate bonds is dangerous now, because colleges can, in a matter of weeks, have to begin repaying millions of dollars of what they had thought of as long-term debt.