J. C. Penney Co. said it plans to buy back up to 15 million shares, or about 10%, of its outstanding common stock. The retailer said in a news release that the purchases will be made on the open market. Chairman William R. Howell said the company's stock was undervalued relative to Penney's financial condition and results, which retail industry analysts have labeled as strong. He said the buyback reflected the belief of the Penney board of directors that the company's performance will continue to be positive. At current prices, the buyback is valued at about $890 million.