A new intrinsic-colour calibration ( ( b – y ) _ { o } – \beta ) is presented for the uvby – \beta photometric system , making use of re-calibrated Hipparcos parallaxes and published reddening maps .
This new calibration for ( b – y ) _ { o } – \beta , our Equation ( 1 ) , has been based upon stars with d _ { Hip } < 70 pc in the photometric catalogues of Schuster et al .
( 1988 , 1993 , 2006 ) , provides a small dispersion , \pm 0.009 , and has a positive “ standard ” +2.239 \Delta \beta coefficient , which is not too different from the coefficients of Crawford ( +1.11 ; 1975a ) and of Olsen ( +1.34 ; 1988 ) .
For 61 stars with spectra from CASPEC , UVES/VLT , and FIES/NOT databases , without detectable Na I lines , the average reddening value \langle E ( b - y ) \rangle = -0.001 \pm 0.002 shows that any zero-point correction to our intrinsic-colour equation must be minuscule .