Metal production in M33: space and time variations
  Nearby galaxies are ideal places to study in detail metallicity gradients and
their time evolution. We consider chemical abundances of a new sample of \hii\
regions complemented with previous literature data-sets. We compare \hii\
region and PN abundances obtained with a common set of observations taken at
MMT. With an updated theoretical model, we follow the time evolution of the
baryonic components and chemical abundances in the disk of M33, assuming that
the galaxy is accreting gas from an external reservoir. Supported by a uniform
sample of nebular spectroscopic observations, we conclude that: {\em i}) the
metallicity distribution in M33 is very complex, showing a central depression
in metallicity probably due to observational bias; {\em ii}) the metallicity
gradient in the disk of M33 has a slope of -0.037$\pm$ 0.009 dex kpc$^{-1}$ in
the whole radial range up to $\sim$8 kpc, and -0.044$\pm$ 0.009 dex kpc$^{-1}$
excluding the central kpc; {\em iii}) there is a small evolution of the slope
with time from the epoch of PN progenitor formation to the present-time.}
