PNe as observational constraints in chemical evolution models for NGC
  6822
  Chemical evolution models are useful for understanding the formation and
evolution of stars and galaxies. Model predictions will be more robust as more
observational constraints are used. We present chemical evolution models for
the dwarf irregular galaxy NGC 6822 using chemical abundances of old and young
Planetary Nebulae (PNe) and \ion{H}{ii} regions as observational constraints.
Two sets of chemical abundances, one derived from collisionally excited lines
(CELs) and one, from recombination lines (RLs), are used. We try to use our
models as a tool to discriminate between both procedures for abundance
determinations. In our chemical evolution code, the chemical contribution of
low and intermediate mass stars is time delayed, while for the massive stars
the chemical contribution follows the instantaneous recycling approximation.
Our models have two main free parameters: the mass-loss rate of a well-mixed
outflow and the upper mass limit, $M_{up}$, of the initial mass function (IMF).
To reproduce the gaseous mass and the present-day O/H value we need to vary the
outflow rate and the $M_{up}$ value.
