September was a month of two halves for the corporate bond markets.In the first half of the month the knock-on effect of problems in the US sub-prime mortgage market continued to dog corporate bonds.Sub-prime lending is the practice of making loans to borrowers who do not qualify for market interest rates because of problems with their credit history and in the US these borrowers continued to default in increasing numbers on their mortgage payments.In the middle of the month the US Federal Reserve took a decisive move to ease the situation and cut interest rates from 5.25% to 4.75%.This helped to restore confidence in the corporate bond market by showing the US central bank's willingness to take whatever measures it deemed necessary to stop the broader economy being affected.