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The apparent size of stars is a crucial benchmark for fundamental stellarproperties such as effective temperatures, radii and surface gravities. Whileinterferometric measurements of stellar angular diameters are the most directmethod to gauge these, they are still limited to relatively nearby and brightstars, which are saturated in most of the modern photometric surveys. Thisdichotomy prevents us from safely extending well calibrated relations to thefaint stars targeted in large spectroscopic and photometric surveys. Here, wealleviate this obstacle by presenting SAAO near-infrared JHK observations of 55stars: 16 of them have interferometric angular diameters, and the rest are incommon with the 2MASS (unsaturated) dataset, allowing us to tie the effectivetemperatures and angular diameters derived via the Infrared Flux Method to theinterferometric scale. We extend the test to recent interferometricmeasurements of unsaturated 2MASS stars, including giants, and the metal-poorbenchmark target HD122563. With a critical evaluation of the systematicsinvolved, we conclude that a 1% accuracy in fundamental stellar parameters isusually within reach. Caution, however, must be used when indirectly testing atemperature scale via colour relations, as well as when assessing thereliability of interferometric measurements, especially at sub-milliarcseclevel. As a result, rather different effective temperature scales can becompatible with a given subset of interferometric data. We highlight somecaveats to be aware of in such a quest, and suggest a simple method to checkagainst systematics in fundamental measurements. A new diagnostic combinationseismic radii with astrometric distances is also presented.

Towards stellar effective temperatures and diameters at one per cent accuracy for future surveys