  The binding energy and wavefunctions of two-dimensional indirect biexcitons
are studied analytically and numerically. It is proven that stable biexcitons
exist only when the distance between electron and hole layers is smaller than a
certain critical threshold. Numerical results for the biexciton binding
energies are obtained using the stochastic variational method and compared with
the analytical asymptotics. The threshold interlayer separation and its
uncertainty are estimated. The results are compared with those obtained by
other techniques, in particular, the diffusion Monte-Carlo method and the
Born-Oppenheimer approximation.
