The missing UV opacity and the colours of solar-type stars

A simple method is proposed to take into account the "missing UV opacity" in solar-type stars. It is shown that the mere inclusion of that UV opacity through a very simple formula is sufficient to bring the theoretical colours in agreement with observed ones for stars of different metal abundances and belonging to the spectral range from mid-F to late-G. Synthetic colours computed in the Geneva and UBV systems reproduce the various observed relations satisfactorily. The relations based on these colours allow reliable estimates of effective temperatures, surface gravities and metal abundances of stars. In addition, solar colours are obtained and three more solar twin candidates are proposed.