  We present near-IR interferometric measurements of the K1.5 giant Arcturus
(alpha Bootis), obtained at the IOTA interferometer with the FLUOR instrument,
in four narrow filters with central wavelengths ranging from 2.03 to 2.39
micron. These observations were expected to allow us to quantify the wavelength
dependence of the diameter of a typical K giant. They are compared to
predictions from both plane-parallel and spherical model atmospheres.
Unexpectedly, neither can explain the observed visibilities. We show how these
data suggest the presence of a companion, in accordance with the Hipparcos data
on this star, and discuss this solution with respect to Arcturus' single star
status.
