The spectral excess theorem for graphs with few eigenvalues whose   distance-$2$ or distance-$1$-or-$2$ graph is strongly regular
  We study regular graphs whose distance-$2$ graph or distance-$1$-or-$2$ graph is strongly regular. We provide a characterization of such graphs $\Gamma$ (among regular graphs with few distinct eigenvalues) in terms of the spectrum and the mean number of vertices at maximal distance $d$ from every vertex, where $d+1$ is the number of different eigenvalues of $\Gamma$. This can be seen as a another version of the so-called spectral excess theorem, which characterizes in a similar way those regular graphs that are distance-regular. 
C. Dalf\'o, M.A. Fiol, J. Koolen
math.CO