  We show that for small values of the chemical potential the curvature of the
phase transition line can be deduced from an analysis of scaling properties of
the chiral condensate and its susceptibilities. We make use of a recent
analysis of the magnetic equation of state in (2+1)-flavor QCD where a
connection between the QCD parameters and the universal scaling fields could be
established. The remaining dependence of the reduced temperature on the
chemical potential can be fixed by an analysis of a mixed susceptibility,
obtained from a derivative with respect to quark mass and chemical potential.
We extract this dependence which describes the curvature of the phase
transition line, at two values of the cut-off, $aT=1/4$ and $1/8$. We find that
cut-off effects are small for the curvature parameter and determine the
transition line in the chiral limit to leading order in the light quark
chemical potential. We obtain $T_c(\mu_B)/T_c(0) = 1 - 0.00656(66) (\mu_B/T)^2
+{\cal O}(\mu_B^4)$.
