Warped compactifications in M and F theory

We study M and F theory compactifications on Calabi-Yau four-folds in the p resence of non-trivial background flux. The geometry is warped and belongs to the class of p-brane metrics. We solve for the explicit warp factor in t he orbifold limit of these compactifications, compare our results to some o f the more familiar recently studied warped scenarios, and discuss the effe cts on the low-energy theory. As the warp factor is generated solely by bac kreaction, we may use topological arguments to determine the massless spect rum. We perform the computation for the case where the four-fold equals K3 x K3. (C) 2000 Elsevier Science B.V. All rights reserved.