Hong Kong stocks fell to five-month lows in thin trading on Tuesday, mainly hurt by weakness in mainland companies listed in the city. The benchmark Hang Seng index extended early losses and fell 0.5 percent, to 21,729.06 points, its lowest close since July 19. The index was down 1.0 percent, at 9,283.41 points. Trading volume was low as investors started to position their portfolios for next year.