  The critical behavior of the Widom-Rowlinson mixture [J. Chem. Phys. 52, 1670
(1970)] is studied in d=3 dimensions by means of grand canonical Monte Carlo
simulations. The finite size scaling approach of Kim, Fisher, and Luijten
[Phys. Rev. Lett. 91, 065701 (2003)] is used to extract the order parameter and
the coexistence diameter. It is demonstrated that the critical behavior of the
diameter is dominated by a singular term proportional to t^(1-alpha), with t
the relative distance from the critical point, and alpha the critical exponent
of the specific heat. No sign of a term proportional to t^(2beta) could be
detected, with beta the critical exponent of the order parameter, indicating
that pressure-mixing in this model is small. The critical density is measured
to be rho*sigma^3 = 0.7486 +/- 0.0002, with sigma the particle diameter. The
critical exponents alpha and beta, as well as the correlation length exponent
nu, are also measured and shown to comply with d=3 Ising criticality.
