Bonds backed by commercial mortgages, with less of a protective cushion than the most senior debt, are rising as investors wager that an economic recovery will stem surging defaults.

Some classes of securities tied to commercial real estate loans, originally rated triple-A and more susceptible to losses than other top-ranked bonds, have gained 30% this year, according to an April 30 report from Barclays Capital. The higher-rated classes among them most likely to take losses, many whose credit standing was already cut, have gained 53%, according to Barclays.

The rally in debt with less insulation from losses "showed little sign of fatigue," Barclays analysts in New York led by Aaron Bryson said.

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