Rolling among G 2 vacua

We consider topology-changing transitions between 7-manifolds of holonomy G2 constructed as a quotient of CY ? S1 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 = 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.