ACA Financial Guaranty Corp. filed suit on January 6, 2011 against Goldman Sachs, alleging fraud and unjust enrichment in connection with the Abacus 2007-AC1 collateralized debt obligation. ACA alleges Goldman designed the securitization to benefit Paulson & Co., who shorted the portfolio, and deceived ACA into believing that Paulson & Co. was an equity investor in the portfolio. ACA insured senior tranches and also invested in the Abacus notes. Complaint.