  The metallicity of the progenitor system producing a type Ia supernova (SN
Ia) could play a role in its maximum luminosity, as suggested by theoretical
predictions. We present an observational study to investigate if such a
relationship there exists. Using the 4.2m WHT we have obtained
intermediate-resolution spectroscopy data of a sample of 28 local galaxies
hosting SNe Ia, for which distances have been derived using methods independent
to those based on the own SN Ia parameters. From the emission lines observed in
their optical spectrum, we derived the gas-phase oxygen abundance in the region
where each SN Ia exploded. Our data show a trend, with a 80% of chance not to
be due to random fluctuation, between SNe Ia absolute magnitudes and the oxygen
abundances of the host galaxies, in the sense that luminosities tend to be
higher for galaxies with lower metallicities. This result seems like to be in
agreement with both the theoretically expected behavior, and with other
observational results. This dependence $M_{B}$-Z might induce to systematic
errors when is not considered in deriving SNe Ia luminosities and then using
them to derive cosmological distances.
