  We investigate the chemical evolution of the Small Magellanic Cloud (SMC)
based on abundance data of planetary nebulae (PNe). The main goal is to
investigate the time evolution of the oxygen abundance in this galaxy by
deriving an age-metallicity relation. Such a relation is of fundamental
importance as an observational constraint of chemical evolution models of the
SMC. We have used high quality PNe data in order to derive the properties of
the progenitor stars, so that the stellar ages could be estimated. We collected
a large number of measured spectral fluxes for each nebula, and derived
accurate physical parameters and nebular abundances. New spectral data for a
sample of SMC PNe obtained between 1999 and 2002 are also presented. These data
are used together with data available in the literature to improve the accuracy
of the fluxes for each spectral line. We obtained accurate chemical abundances
for PNe in the Small Magellanic Cloud, which can be useful as tools in the
study of the chemical evolution of this galaxy and of Local Group galaxies. We
present the resulting oxygen versus age diagram and a similar relation
involving the [Fe/H] metallicity based on a correlation with stellar data. We
discuss the implications of the derived age-metallicity relation for the SMC
formation, in particular by suggesting a star formation burst in the last 2-3
Gyr.
