Citigroup Inc chairman and chief executive officer (CEO) Charles Prince, beset by the company's billions of dollars in losses from investing in bad debt, has retired and is being replaced as chairman by former US treasury secretary Robert Rubin, the company said.In an announcement yesterday following an emergency meeting of Citi's board, the United States' largest banking company also said Sir Win Bischoff, chairman of Citi Europe and a member of the Citi management and operating committees, would serve as an interim CEO.Rubin, a former co-chairman of Goldman, Sachs & Co, has served as the chair of Citi's executive committee.Prince's resignation, which was secured at an emergency meeting of the Citi board yesterday, was expected after the company revealed it had to write down billions of dollars in bad debt. He joined former Merrill Lynch & Co CEO Stan O'Neal, who resigned from the investment bank last month, as the highest-profile casualties of the debt crisis that has cost billions at other financial institutions as well.In a separate statement, Citi said it would take an additional $8-11 billion in writedowns. It has already said it was writing down $6.5 billion in assets.Prince, 57, became chief executive of Citigroup in October 2003. Many shareholders criticised him openly for much of his tenure as Citigroup's stock lagged its peers while Prince executed what was called an umbrella model of corporate organisation with several separate lines of business.