Head-on collisions of unequal mass black holes: Close-limit predictions

The close-limit method has given approximations in excellent agreement with those of numerical relativity for collisions of equal mass black holes. We consider here colliding holes with unequal mass, for which numerical relativity results are not available. We try to ark two questions. (i) Can we get approximate answers to astrophysical questions (ideal mass ratio for energy production, maximum recoil velocity, etc.)? (ii) Can we better understand the limitations of approximation methods? Then is some success in answering the first type of question, but more with the second, especially in connection with the issue of measures of the intrinsic mass of the colliding holes, and of the range of validity of the method. [S0556-2821(97)00622-X].