  We present results on the curvature of the critical line of QCD with 2+1 HISQ
fermions at nonzero temperature and quark density obtained by analytic
continuation from imaginary chemical potentials. Monte Carlo simulations are
performed by means of the MILC code, suitably modified to include a nonzero
imaginary baryon chemical potential. We set the chemical potential at the same
value for the three quark species and work on the line of constant physics with
a light to strange mass ratio of 1/20 as determined in
Ref.~\cite{Bazavov:2011nk}.
