Time variation of the O/H radial gradient in the galactic disk based on
  planetary nebulae
  The controversy on the time variation of the radial abundance gradients can
in principle be settled by estimating the gradients from planetary nebulae (PN)
ejected by central stars (CSPN) with different ages. In this work, we consider
four samples of CSPN whose lifetimes have been estimated using three different
methods and estimate the oxygen abundance gradients for these objects. The
results suggest some small differences between the younger and older CSPN. The
younger objects have similar or slightly higher oxygen abundances compared with
the older objects, and the gradients of both groups are similar within the
uncertainties. Therefore, the O/H radial gradient has not changed appreciably
during the lifetime of the objects considered, so that PN gradients are not
expected to be very different from the gradients observed in younger objects,
which seems to be supported by recent observational data.
