DOW JONES NEWSWIRES Southwest Airlines Co. (LUV) said May traffic at its namesake airline rose 10.7% while its recently acquired AirTran posted a 10.1% traffic increase, adding strength to recent growth trends for the discount carriers. Southwest, the largest domestic U.S. airline by passenger numbers, closed its $1.4 billion takeover of discount carrier AirTran last month in a bid to expand its reach and capacity. Southwest has also revamped its frequent-flier program as it aims to transform itself from a discount carrier to a more diversified airline business. For May, Southwest estimated its passenger revenue per available seat mile rose 11% to 12% over the prior year, excluding AirTran. Capacity at the namesake carrier grew 3.9%, while load factor--the percentage of available seats filled--rose to 82.3% from 77.2%. AirTran's traffic figures also continued to pick up steam, while the company's vigorous capacity increases again meant planes weren't quite as full as a year earlier. AirTran's traffic growth represented the fourth-straight month of accelerating growth rates. Capacity rose 5.3%, while load factor rose to 84.5% from 80.8%. In early trading Tuesday, shares of Southwest rose 12 cents to $11.35. The stock is off 12% this year. -By Drew FitzGerald, Dow Jones Newswires; 212-416-2909; Andrew.FitzGerald@dowjones.com