  Context: The prediction of stellar angular diameters from broadband
photometry plays an important role for different applications. In particular,
long-baseline interferometry, gravitational microlensing, extrasolar planet
transits, and many other observing techniques require accurate predictions of
the angular size of stars. These predictions are based on the surface
brightness-colour (SBC) relations. Aims: Our goal is to calibrate
general-purpose SBC relations using visible colours, the most commonly
available data for most stars. Methods: We compiled the existing long-baseline
interferometric observations of nearby dwarf and subgiant stars and the
corresponding broadband photometry in the Johnson B V and Cousins Rc Ic bands.
We then adjusted polynomial SBC models to these data. Results: Due to the
presence of spectral features that depend on the effective temperature, the SBC
relations are usually not linear for visible colours. We present polynomial
fits that can be employed with BVRcIc based colours to predict the
limb-darkened angular diameters (i.e. photospheric) of dwarf and subgiant stars
with a typical accuracy of 5%. Conclusions: The derived polynomial relations
provide a satisfactory approximation to the observed surface brightness of
nearby dwarfs and subgiants. For distant stars, the interstellar reddening
should be taken into account, and will usually introduce an additional
uncertainty to the predicted angular diameters.
