We present direct measurements of the limb-darkened intensity profiles of the late-type giant stars HR 5299 , HR 7635 , and HR 8621 obtained with the Navy Prototype Optical Interferometer ( NPOI ) at the Lowell Observatory .
A triangle of baselines with lengths of 18.9 m , 22.2 m , and 37.5 m was used .
We utilized squared visibility amplitudes beyond the first minimum , as well as triple amplitudes and phases in up to 10 spectral channels covering a wavelength range of \sim 650 nm to \sim 850 nm .
We find that our data can best be described by featureless symmetric limb-darkened disk models while uniform disk and fully darkened disk models can be rejected .
We derive high-precision angular limb-darkened diameters for the three stars of 7.44 mas \pm 0.11 mas , 6.18 mas \pm 0.07 mas , and 6.94 mas \pm 0.12 mas , respectively .
Using the HIPPARCOS parallaxes , we determine linear limb-darkened radii of 114 R _ { \odot } \pm 13 R _ { \odot } , 56 R _ { \odot } \pm 4 R _ { \odot } , and 98 R _ { \odot } \pm 9 R _ { \odot } , respectively .
We compare our data to a grid of Kurucz stellar model atmospheres , with them derive the effective temperatures and surface gravities without additional information , and find agreement with independent estimates derived from empirical calibrations and bolometric fluxes .
This confirms the consistency of model predictions and direct observations of the limb-darkening effect .