Context :

Aims : In this paper we demonstrate how Strömgren uvby photometry can be efficiently used to : 1 .
Identify red giant branch stars that are members in a dwarf spheroidal galaxy .
2 .
Derive age-independent metallicities for the same stars and quantify the associated errors .

Methods : Strömgren uvby photometry in a 11 \times 22 arcmin field centered on the Draco dwarf spheroidal galaxy was obtained using the Isaac Newton Telescope on La Palma .
Members of the Draco dSph galaxy were identified using the surface gravity sensitive c _ { 1 } index which discriminates between red giant and dwarf stars .
Thus enabling us to distinguish the ( red giant branch ) members of the dwarf spheroidal galaxy from the foreground dwarf stars in our galaxy .
The method is evaluated through a comparison of our membership list with membership classifications in the literature based on radial velocities and proper motions .
The metallicity sensitive m _ { 1 } index was used to derive individual and age-independent metallicities for the members of the Draco dSph galaxy .
The derived metallicities are compared to studies based on high resolution spectroscopy and the agreement is found to be very good .

Results : We present metallicities for 169 members of the red giant branch in the Draco dwarf spheroidal galaxy ( the largest sample to date ) .
The metallicity distribution function for the Draco dSph galaxy shows a mean [ Fe/H ] = –1.74 dex with a spread of 0.24 dex .
The correlation between metallicity and colour for the stars on the red giant branch is consistent with a dominant old , and coeval population .
There is a possible spatial population gradient over the field with the most metal-rich stars being more centrally concentrated than the metal-poor stars .

Conclusions :