  We say that a Cellular Automata (CA) is coalescing when its execution on two
distinct (random) initial configurations in the same asynchronous mode (the
same cells are updated in each configuration at each time step) makes both
configurations become identical after a reasonable time.
  We prove coalescence for two elementary rules, non coalescence for two other,
and show that there exists infinitely many coalescing CA. We then conduct an
experimental study on all elementary CA and show that some rules exhibit a
phase transition, which belongs to the universality class of directed
percolation.
