The New York Stock Exchange halted trading on both Ambac Financial Group Inc. and Radian Group Inc. under its "sub-penny" rules yesterday after shares of both insurers fell below $1.05.

The exchange stops trading on a security when it is - or is "imminently likely" - to trade below $1.00. The sub-penny halt is automatically triggered when a shares of a stock fall below $1.05, according to NYSE rules.

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