  We investigate a possible correlation between the orbital periods $P$ of the
extra-solar planet sample and the metallicity [Fe/H] of their parent stars.
Close-in planets, on a few-days orbits, are more likely to be found around
metal-rich stars. Simulations show that a weak correlation is present. This
correlation becomes stronger when only sigle stars with one detected planet are
considered. We discuss several potential sources of bias that might mimic the
correlation, and find they can be ruled out, but not with high significance. If
real, the absence of very short-period planets around the stellar sample with
[Fe/H] $< 0.0$ can be interpreted as evidence of a metallicity dependence of
the migration rates of giant planets during formation in the protoplanetary
disc. The observed $P-$[Fe/H] correlation can be falsified or confirmed by
conducting spectroscopic or astrometric surveys of metal-poor stars ([Fe/H] $<
-0.5$) in the field.
