  String consistency conditions are stronger than anomaly cancellation and can
require the addition of exotics in the visible sector. We study such exotics
and demonstrate that they may account for the modest excess at $750$ GeV in
recent diphoton resonance searches performed by the ATLAS and CMS
collaborations. In a previous analysis of type II MSSM D-brane quivers we
systematically added up to five exotics for the sake of satisfying string
consistency conditions. Using this dataset, we demonstrate that 89780 of the
89964 quivers have exotics, 78155 of which include singlets that may couple to
MSSM or exotic multiplets with coupling structures governed by $U(1)$
symmetries that are often anomalous. We demonstrate that certain sets of
exotics are far preferred over others and study the structure of singlet
couplings to heavy exotics carrying standard model charges. Typical
possibilities include singlets that may decay to vector-like quarks and / or
vector-like leptons and subsequently to two photons. We show that a narrow
width diphoton excess can be accounted for while evading existing bounds if
multiple exotics are added, with vector-like leptons of mass $M_L\lesssim 375$
GeV and vector-like quarks with masses up to $\simeq 3$ TeV. However, a large
width $(\Gamma/M \sim 0.06)$, as suggested by the ATLAS data, cannot be easily
accommodated in this framework. Renormalization group equations with GUT-scale
boundary conditions show that these supersymmetric models are perturbative and
stable. Type IIA compactifications on toroidal orbifolds allow for $O(10)$
Yukawa couplings in the ultraviolet. We also discuss the possibility of
accounting for the diphoton excess in a low string scale scenario via the decay
of string axions.
