A handful of managers are tweaking RMBS deal structures in order to achieve a Moody's Investors Service triple-A rating on super senior tranches they would not be able to obtain otherwise. The structural nuances essentially burrow the so-called senior support tranche into a more subordinated position in order to protect the super senior tranche. At least for the deals that have been rated so far, the senior support tranche and other senior interest would not receive a triple-A rating per Moody's rating methodology, but that's not to say the manager could not shop around for one.

While the rating agency would not confirm, it would appear as though managers are bypassing a lower rating from Moody's for the senior support tranche and other senior classes for triple-A ratings achieved elsewhere - a move that could be confusing for investors who think all triple-As, and senior classes for that matter, are created equal. Moody's rated four or five such deals in the fourth quarter, and inquiries continue to trickle in. "Senior class investors need to be aware of these differences," said Joseph Grohotolski, a vice president at Moody's.

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