  A Roman dominating function (RD-function) on a graph $G = (V(G), E(G))$ is a
labeling $f : V(G) \rightarrow \{0, 1, 2\}$ such that every vertex with label
$0$ has a neighbor with label $2$. The weight $f(V(G))$ of a RD-function $f$ on
$G$ is the value $\Sigma_{v\in V(G)} f (v)$. The {\em Roman domination number}
$\gamma_{R}(G)$ of $G$ is the minimum weight of a RD-function on $G$. The six
classes of graphs resulting from the changing or unchanging of the Roman
domination number of a graph when a vertex is deleted, or an edge is deleted or
added are considered. We consider relationships among the classes, which are
illustrated in a Venn diagram. A graph $G$ is Roman domination $k$-critical if
the removal of any set of $k$ vertices decreases the Roman domination number.
Some initial properties of these graphs are studied. The $\gamma_R$-graph of a
graph $G$ is any graph which vertex set is the collection $\mathscr{D}_R(G)$ of
all minimum weight RD-functions on $G$. We define adjacency between any two
elements of $\mathscr{D}_R(G)$ in several ways, and initiate the study of the
obtained $\gamma_R$-graphs.
