An estimate of the time variation of the abundance gradient from planetary nebulae

The time behaviour of the radial abundance gradients in the galactic disk is investigated on the basis of four different samples of planetary nebulae, comprising both smaller, homogeneous sets of data and larger, albeit non-homogeneous, samples. Four different chemical elements are considered, namely, oxygen, sulphur, argon, and neon. Our analysis supports our earlier conclusions that, on the average, the radial abundance gradients have flattened out in the last 6 to 8 Gyr