Study of the transverse mass spectra of strange particles in
Pb-Pb collisions at the SPS.

Experiment NA57 has collected high purity samples of \PKzS\ and \PgL, $\Xi$\
and $\Omega$\ hyperons produced in Pb-Pb collisions at both 40 and 158 $A$\
GeV/$c$.  It will be presented a complete study of the transverse mass
spectra of these particles for the full sample of events corresponding to
about the most central 53\% of the inelastic Pb-Pb cross section. The
inverse slopes of the spectra will be presented and discussed, also as a
function of centrality. We analyse the transverse mass distributions in the
framework of the blast-wave model as a function of centrality, energy and
particle species. Different transverse flow profiles will be considered and
compared. Thermal freeze-out conditions (i.e. temperature and transverse
flow velocity) will be compared to those measured by other experiments, e.g.
at RHIC energies.