Royal Bank of Scotland (RBS) is in talks with private equity groups to boost the balance sheet of its Ulster Bank unit.Private equity groups Warburg Pincus, CVC, KKR and Permira are all said to be looking into a proposal to inject money into the Irish business before it is merged with another Irish bank, according to the Sunday Times. TSB, owned by the Irish bank, is considered the most likely partner for a newly merged bank.The deal would allow RBS to reduce its holding in Ulster below 50%, and could help overcome the hurdle to selling some of the government's majority stake. Ulster has accumulated up £2.5bn of losses over the past two years, but it rebounded in the first quarter with £17m profit. RD