SEOUL, Dec. 9 (Yonhap) -- Affiliates of Doosan Group, a machinery and construction equipment conglomerate, saw their credit ratings downgraded by local rating appraisers due to lingering concerns over their financial health despite the market debut of a key affiliate, industry sources said Friday. Doosan Bobcat Inc., a unit of the country's leading construction equipment maker Doosan Infracore, raised some 900 billion won (US$773 million) through the IPO last month, the second-largest IPO deal after Samsung BioLogics Co.'s 2.2 trillion-won market debut early this month. Originally, Doosan Bobcat, a leading player in the small construction machinery sector in the United…