Rolling among G2 vacua

We consider topology-changing transitions between 7-manifolds of holonomy G2 constructed as a quotient of CYxS1 by an antiholomorphic involution. We classify involutions for Complete Intersection CY threefolds, focussing primarily on cases without fixed points. The ordinary conifold transition between CY threefolds descends to a transition between G2 manifolds, corresponding in the N=1 effective theory incorporating the light black hole states either to a change of branch in the scalar potential or to a Higgs mechanism. A simple example of conifold transition with a fixed nodal point is also discussed. As a spin-off, we obtain examples of G2 manifolds with the same value for the sum of Betti numbers b2+b3, and hence potential candidates for mirror manifolds. (author)