NEW YORK — LaBranche & Co., one of the largest specialist firms on the New York Stock Exchange, confirmed plans Wednesday to buy a smaller specialist firm, Robb Peck McCooey Financial Services, for more than $286 million of stock.

LaBranche said in a statement that it would offer 9.7 million shares of its common stock and an unspecified number of shares of preferred stock for the privately held firm. Due diligence was not yet completed, the statement said.

Robb Peck’s specialist unit, RPM Specialist Corp., is the sixth-largest New York exchange specialist firm as measured by listings and the seventh-largest by trading volume. Its specialists trade 137 companies, including Wells Fargo & Co. The company also has a clearing operation.

The statement also said the deal would create a firm that would make markets for 534 companies and would control 27% of the dollar volume on the Big Board.

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