Which client segment consumed the least in September 2013?
September 2013 refers to yearmonth.date = '201309'
SELECT T1.Segment FROM customers AS T1 INNER JOIN yearmonth AS T2 ON T1.CustomerID = T2.CustomerID WHERE T2.Date = '201309' GROUP BY T1.CustomerID ORDER BY SUM(T2.Consumption) ASC LIMIT 1