  The degree-diameter problem seeks to find the maximum possible order of a
graph with a given (maximum) degree and diameter. It is known that graphs
attaining the maximum possible value (the Moore bound) are extremely rare, but
much activity is focused on finding new examples of graphs or families of graph
with orders approaching the bound as closely as possible.
  There has been recent interest in this problem as it applies to mixed graphs,
in which we allow some of the edges to be undirected and some directed. A 2008
paper of Nguyen and Miller derived an upper bound on the possible number of
vertices of such graphs. We show that for diameters larger than three, this
bound can be reduced and we present a corrected Moore bound for mixed graphs,
valid for all diameters and for all combinations of undirected and directed
degrees.
