Duff & Phelps (DUF.N) said the firms will pay $15.55 a share to stockholders. The other buyers in the consortium are Stone Point Capital, Pictet & Cie and Edmond de Rothschild Group.

The buyers are offering a premium of 19.2 percent for the company, which closed at $13.05 a share on Friday.

The deal allows Duff & Phelps Corp. a "go-shop" period starting immediately and ending on February 8, 2013, during which it will seek higher offers from other potential buyers.

Centerview Partners is advising Duff & Phelps on the deal, while Sandler O'Neill and Partners, Credit Suisse, Barclays, and RBC Capital Markets are advising the private equity firms.

The agreement includes a break-up fee of $6.65 million from Duff & Phelps if the company abandons the deal for a higher offer before March 8, 2013.

Duff and Phelps Corp. advises clients on areas such as valuation, transactions, financial restructuring, alternative assets, disputes and taxation. It employs more than 1,000 people and has offices in North America, Europe, and Asia.

(Reporting by Sam Forgione; Editing by Jan Paschal)

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