  We review our knowledge of the phase diagram of QCD as a function of
temperature, chemical potential and quark masses. The presence of tricritical
lines at imaginary chemical potential mu=i pi/3 T, with known scaling behaviour
in their vicinity, puts constraints on this phase diagram, especially in the
case of two light flavors. We show first results in our project to determine
the finite-temperature behaviour in the two-flavour chiral limit.
