  The CERN SuperProtoSynchrotron (SPS) represents an ideal facility for
fixed-target heavy-ion experiments exploring the phase diagram of strongly
interacting matter in the region $200\le\mu_{\rm B}\le500$ MeV. It can deliver
high-intensity beams ($>10^6$ Pb/s), allowing a study of rare probes of the
Quark-Gluon Plasma, including electromagnetic and hard processes. The
NA61/SHINE experiment is currently active and plans to perform a first direct
measurement of open charm production in Pb--Pb collisions at top SPS energy and
possibly at lower energies. The project of a new experiment, NA60+, based on a
muon spectrometer coupled to a vertex spectrometer is currently being
developed, for the study of dimuon and heavy quark production, and a Letter of
Intent was recently submitted. In this contribution the physics motivation for
the studies of rare probes, the existing and planned experimental set-ups and
their expected physics performance will be discussed.
