The cobondage number of a graph

A set D of vertices in a graph G = (V, E) is a dominating set of G if every vertex in V −D is adjacent to some vertex in D. The domination number γ(G) of G is the minimum cardinality of a dominating set. We define the cobondage number bc(G) of G to be the minimum cardinality among the sets of edges X ⊆ P2(V ) −E, where P2(V ) = {X ⊆ V : |X| = 2} such that γ(G + X) < γ(G). In this paper, the exact values of bc(G) for some standard graphs are found and some bounds are obtained. Also, a Nordhaus-Gaddum type result is established.