@article{article5df65ddb,
title   = {An estimate of the time variation of the O/H radial gradient from planetary nebulae},
author = {Maciel, Walter Junqueira and Costa, Roberto Dell'Aglio Dias da and Uchida, M.M.M},
year = {2003},
doi = {10.1051/0004-6361:20021530},
journal   = {Astronomy and Astrophysics}
}