Comparison of stellar angular diameters from the NPOI, the Mark III optical interferometer, and the infrared flux method

The Navy Prototype Optical Interferometer (NPOI) has been used to measure t he angular diameters of 41 late-type giant and supergiant stars previously observed with the Mark III optical interferometer. Sixteen of these stars h ave published angular diameters based on model atmospheres (infrared flux m ethod, IRFM). Comparison of these angular diameters shows that there are no systematic offsets between any pair of data sets. Furthermore, the reporte d uncertainties in the angular diameters measured using both interferometer s are consistent with the distribution of the differences in the diameters. The distribution of diameter differences between the interferometric and m odel atmosphere angular diameters are consistent with uncertainties in the IRFM diameters of 1.4%. Although large differences in angular diameter meas urements are seen for three stars, the data are insufficient to determine w hether these differences are due to problems with the observations or are d ue to temporal changes in the stellar diameters themselves.