The evolutionary state of the Beta Canis Majoris variables - I. Accurate beta photometry of early B stars

New β photometry is presented for all the known β Canis Majoris variables and for other bright early B stars observable from the southern hemisphere which were close to the β CMa stars in a β/[c₁] diagram published earlier. The new β values are accurate to $$\pm 0.002 \enspace \text{or}\enspace 0.003$$ mag and enable the ‘instability strip’ along which the variables lie to be defined much more precisely. Several of the other B stars also lie in the strip; most of these have already been found to be non-variable in a subsidiary observing programme.