the critical behavior is compatible with the directed percolation (DP) universality class; this conclusion is also supported by field-theoretic arguments [@scp].

In [@scp], we studied the model with symmetrical rates under exchange of species labels A and B, i.e., with $\lambda_A=\lambda_B=\lambda$. We found that for $\mu < 1$ the transition from the fully active to the absorbing phase occurs at some $\lambda_{c} (\mu) < \lambda_{c} (\mu=1)$, since the annihilation rate is reduced. The effect of asymmetric creation rates is shown in Fig. 1 : if one of the species, for instance A, has its creation rate below (above) $\lambda_c$, the transition occurs for a $\lambda_B$ above (below) $\lambda_c$. (The simulation algorithm is detailed in Sec. IV.) The results for $d=2$ are qualitatively the same, as shown in Fig. 2. Suppose we let $\lambda_A \to \infty$. Then all sites will bear an A particle, so that the dynamics of species B is a contact process with death rate $\mu$. It follows that the critical value of $\lambda_B$ is $\mu \, \lambda_c (\mu=1)$; this determines the asymptotic form of the phase boundaries in Figs. 1 and 2. The simulation data in Figs. 1 and 2 are obtained by extrapolating moment ratio crossings 