  We consider the two sequences of biorthogonal polynomials (p_{k,n})_k and
(q_{k,n})_k related to the Hermitian two-matrix model with potentials V(x) =
x^2/2 and W(y) = y^4/4 + ty^2. From an asymptotic analysis of the coefficients
in the recurrence relation satisfied by these polynomials, we obtain the
limiting distribution of the zeros of the polynomials p_{n,n} as n tends to
infinity. The limiting zero distribution is characterized as the first measure
of the minimizer in a vector equilibrium problem involving three measures which
for the case t=0 reduces to the vector equilibrium problem that was given
recently by two of us. A novel feature is that for t < 0 an external field is
active on the third measure which introduces a new type of critical behavior
for a certain negative value of t. We also prove a general result about the
interlacing of zeros of biorthogonal polynomials.
