TOKYO, Oct 19 (Reuters) - Japan’s benchmark 10-year government bond yields fell on Tuesday, tracking U.S. Treasury yields, while a domestic 20-year notes auction received its lowest book-cover ratio in six years. The 10-year JGB yield fell 0.5 basis point to 0.085%, while the 20-JGB yield was unchanged at 0.470%. The auction for 20-year bonds received bids worth 2.69 times the amount sold, the lowest bid-cover ratio since 2015. The previous sale for a bond with the same maturity saw a bid-cover ratio of 3.2 times. U.S. Treasury yields also edged lower in Asia, particularly in the middle of the curve. The 30-year JGB yield fell 0.5 basis point to 0.690%, while the 40-year JGBs were untraded and the yield stayed at 0.765%. Both the two-year JGB yield and the five-year yield were unchanged at minus 0.120% and minus 0.075%, respectively. Benchmark 10-year JGB futures rose 0.06 point to 151.34, with a trading volume of 18,100 lots. (Reporting by Tokyo markets team)