A group of private equity firms, including the Carlyle Group , struck a deal on Sunday to buy financial advisory and investment banking firm Duff & Phelps Corp for about $665.5 million.

Duff & Phelps 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 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.

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)