  We determine the (pseudo)critical lines of QCD with two degenerate staggered
fermions at nonzero temperature and quark or isospin density, in the region of
imaginary chemical potentials; analytic continuation is then used to prolongate
to the region of real chemical potentials. We obtain an accurate determination
of the curvatures at zero chemical potential, quantifying the deviation between
the case of finite quark and of finite isospin chemical potential. Deviations
from a quadratic dependence of the pseudocritical lines on the chemical
potential are clearly seen in both cases: we try different extrapolations and,
for the case of nonzero isospin chemical potential, confront them with the
results of direct Monte Carlo simulations. Finally we find that, as for the
finite quark density case, an imaginary isospin chemical potential can
strengthen the transition till turning it into strong first order.
