(Reuters) - Copper edged lower for a third session on Wednesday as concern about the outlook for global demand persisted despite the possibility that the U.S. central bank may announce measures to stimulate economic growth at a meeting this week. U.S. Federal Reserve Chairman Ben Bernanke will meet with other central bankers from around the world at Jackson Hole, Wyoming, on Friday, and is expected to detail steps to revive the world's largest economy. Hopes of stimulus measures have boosted gold prices, with bullion staying near 4-1/2-month highs, but copper investors were less convinced. "Sentiment is bearish, with some people questioning the effectiveness of stimulus measures by the U.S. even if they do manage to roll out such measures this week," said Orient Futures derivatives director Andy Du. Three-month copper on the London Metal Exchange dropped 0.5 percent to $7,568 per tonne by 0711 GMT. The most active December copper contract on the Shanghai Futures Exchange slipped 0.5 percent to close at 55,210 yuan ($8,700) per tonne. Sentiment during the session was also dragged down by the Shanghai stock market .SSEC, which logged its lowest close since February 2009, said CIFCO Futures analyst Zhou Jie. Traders said losses in Shanghai copper were limited by increased buying in the physical markets as shown in the prompt September contract trading at a premium to forward months. "Traders have put more copper into ShFE warehouses to try to take advantage of Shanghai copper's forward curve backwardation," said a Shanghai-based physical trader. Copper stocks on warehouses monitored by the Shanghai Futures Exchange rose by nearly 3,000 tonnes to 158,938 tonnes last week. LME tin was the biggest loser of the day, dropping 2.9 percent at one point to a session low of 20,100 after Indonesian tin producer PT Timah (TINS.JK) said it has re-started spot sales following a three-week stoppage. Prices had shot up 13 percent last week on news that over 90 percent of Indonesian tin miners have stopped production.

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