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Synthetic spectra of Arcturus computed with the latest theoretical stellar atmospheres disagree strongly with both the spectral energy distribution and with the spectroscopic data of Griffin 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. CCD spectroscopy of Arcturus and two other K2 III stars shows that the spectrum of Arcturus in not anomalous and that in the violet region the calibration of the spectral atlas of Arcturus of Griffin is accurate. Therefore, missing opacity may be a common feature of the models for stars in this temperature and luminosity range.
