  Classical Jacobi polynomials $P_{n}^{(\alpha,\beta)}$, with $\alpha,
\beta>-1$, have a number of well-known properties, in particular the location
of their zeros in the open interval $(-1,1)$. This property is no longer valid
for other values of the parameters; in general, zeros are complex. In this
paper we study the strong asymptotics of Jacobi polynomials where the real
parameters $\alpha_n,\beta_n$ depend on $n$ in such a way that $$
\lim_{n\to\infty}\frac{\alpha_{n}}{n}=A, \quad
\lim_{n\to\infty}\frac{\beta_{n}}{n}=B, $$ with $A,B \in \mathbb{R}$. We
restrict our attention to the case where the limits $A,B$ are not both positive
and take values outside of the triangle bounded by the straight lines A=0, B=0
and $A+B+2=0$. As a corollary, we show that in the limit the zeros distribute
along certain curves that constitute trajectories of a quadratic differential.
  The non-hermitian orthogonality relations for Jacobi polynomials with varying
parameters lie in the core of our approach; in the cases we consider, these
relations hold on a single contour of the complex plane. The asymptotic
analysis is performed using the Deift-Zhou steepest descent method based on the
Riemann-Hilbert reformulation of Jacobi polynomials.
