  The NA57 experiment has collected high statistics, high purity samples of
\PKzS and \PgL, $\Xi$ and $\Omega$ hyperons produced in Pb-Pb collisions at 158
$A$ GeV/$c$. In this paper we present a study of the transverse mass spectra of
these particles for a sample of events corresponding to the most central 53% of
the inelastic Pb-Pb cross-section. We analyse the transverse mass distributions
in the framework of the blast-wave model for the full sample and, for the first
time at the SPS, as a function of the event centrality.
