Luann de Lesseps wasn't just betrayed by her former hubby Tom D'Agostino! Breaking Celebrity News can exclusively reveal that her former publicist pal, Anna Rothschild, is D'Agostino's latest mystery blonde!

D'Agostino, 50, and Rothschild, 52, were caught in the Hamptons together over the Labor Day weekend, while de Lesseps, 52, was alone in Sag Harbor.

Rothschild and de Lesseps were friends for years prior to the RHONY star's New Year's Eve wedding , and her nuptials could have ended their relationship. Their last photo together was a year prior to the Palm Beach, Fla. celebration.