We present BV photometry of the Sculptor dwarf galaxy to V = 22 .
These data give evidence for a bimodality in Sculptor ’ s metallicity distribution based on a discontinuity in the luminosities of horizontal branch ( HB ) stars and by the presence of two distinct red giant branch ( RGB ) bumps .
A consistent picture of the evolved stars in Sculptor is given by the presence of ( 1 ) a metal-poor population of { [ Fe / H ] } \sim - 2.3 with an exclusively blue HB and that corresponds to the blueward side of the Sculptor RGB and the more luminous RGB bump , and ( 2 ) a less metal-poor population of { [ Fe / H ] } \sim - 1.5 required to explain the less luminous red HB , the red side of the RGB , and a second , less luminous RGB bump .
Best fits to the HB populations are obtained with enhanced oxygen abundances , { [ O / Fe ] } \sim + 0.5 .
Variations in the global HB and RGB morphology of Sculptor can be explained by differences in the radial distribution of these two populations .
The presence of these two populations shows that the Sculptor dwarf galaxy has an internal second parameter problem .