  Synthetic spectra of Arcturus computed with the latest theoretical stellar
atmospheres disagree strongly with both the spectral energy distribution and
with the spectroscopic data for $\lambda < 4000$ \AA , even though the stellar
parameters are well constrained. We find that the discrepancy can be removed by
adding a continuous absorption opacity that is approximately equal to that
which is already included in the {\sc atlas9} code. Spectroscopy of other
  K2 III stars shows that the spectrum of Arcturus in not anomalous. Therefore,
missing opacity may be a common feature of the models for stars in this
temperature and luminosity range.
