Theoretical TO absolute magnitudes from the D07 stellar evolution library. Lines represent M ^TO _{F606 W} for different ages as a function of global metallicity, in 1 Gyr (solid lines) and 0.5 Gyr (dashed lines) age steps. The lower isochrone corresponds to an age of 15 Gyr. Points represent the measured MSTO magnitudes. The upper panel shows the M ^TO _{F606 W} magnitude as a function of the metallicity in the ZW scale; the lower panel gives M ^TO _{F606 W} as a function of the metallicity in the CG scale.