Chemical abundances of planetary nebulae towards the direction of the   galactic anticenter
  In this paper we report new observations and derive chemical abundances for a sample of 26 planetary nebulae (PN) located in the anticenter direction. Most of these nebulae are far away objects, located at galactocentric distances greater than about 8 kpc, so that they are particularly useful for the determination of the radial gradients at large distances from the galactic center. A comparison of the present results with previously determined abundances suggests that the radial abundance gradients flatten out at distances larger than about 10 kpc from the center. 
R.D.D. Costa, M.M.M. Uchida, W.J. Maciel