The number of mortgage applications filed this past week edged up a slight 0.6% as more Americans  refinanced their mortgages, an industry trade group said Wednesday. For the  week ending Sept. 16, the Mortgage Bankers Association found the market composite index -- a measure of mortgage loan application volume -- grew 0.6% on a seasonally adjusted basis. Much of the limited positive activity was driven by homeowners refinancing existing loans. The refinance index grew 2.2% over the previous week, while the seasonally adjusted purchase index fell 4.7% from a week earlier. The MBA report says the refinancing share of mortgage activity in the U.S. represented 78.3% of all applications filed, compared to 76.8% a week earlier. The ARM share of activity fell from 7.3% to 6.7%. The MBA released new data showing investors represented 5.7% of all homes purchased in August, up from 5.5% in July. New investor activity in the Pacific region also drove the increase, the MBA said. Overall, the share of mortgage purchases on second homes edged up from 5.9% in July to 6% in August. When looking at the average contract rate on a 30-year, fixed-rate mortgage with a conforming loan balance of $417,500 or less, the MBA found the rate  remained the same, hovering at 4.29% in August. The average contract interest rate for 30-year FRMs tied to jumbo loans fell to 4.55% from 4.57%. Looking at the 15-year FRM, the average rate declined to 3.46% from 3.52%. The MBA reported "the average contract interest rate for 30-year fixed-rate mortgages backed by the FHA  decreased to 4.07 percent from 4.08 percent, with points increasing to 0.51 from 0.48 (including the origination fee) for 80 percent loan-to-value (LTV) ratio loans." Meanwhile, 5/1 ARMs had an average interest rate of 2.96 this past week, down from 2.99%  a week earlier. Write to Kerri Panchuk.