Context : dSph galaxies are the least luminous , least massive galaxies known .
Recently , the number of observed galaxies in this class has greatly increased thanks to large surveys .
Determining their properties , such as mass , luminosity and metallicity , provides key information in our understanding of galaxy formation and evolution .

Aims : Our aim is to provide as clean and as complete a sample as possible of red giant branch stars that are members of the Hercules dSph galaxy .
With this sample we explore the velocity dispersion and the metallicity of the system .

Methods : Strömgren photometry and multi-fibre spectroscopy are combined to provide information about the evolutionary state of the stars ( via the Strömgren c _ { 1 } index ) and their radial velocities .
Based on this information we have selected a clean sample of red giant branch stars , and show that foreground contamination by Milky Way dwarf stars can greatly distort the results .

Results : Our final sample consists of 28 red giant branch stars in the Hercules dSph galaxy .
Based on these stars we find a mean photometric metallicity of -2.35 \pm 0.31 dex which is consistent with previous studies .
We find evidence for an abundance spread .
Using those stars for which we have determined radial velocities we find a systemic velocity of 45.20 \pm 1.09 { km s ^ { -1 } } with a dispersion of 3.72 km s ^ { -1 } , this is lower than values found in the literature .
Furthermore we identify the horizontal branch and estimate the mean magnitude of the horizontal branch of the Hercules dSph galaxy to be V _ { 0 } = 21.17 \pm 0.05 , which corresponds to a distance of 147 ^ { +8 } _ { -7 } kpc .

Conclusions : When studying sparsely populated and/or heavily foreground contaminated dSph galaxies it is necessary to include knowledge about the evolutionary stage of the stars .
This can be done in several ways .
Here we have explored the power of the c _ { { 1 } } index in Strömgren photometry .
This index is able to clearly identify red giant branch stars redder than the horizontal branch , enabling a separation of red giant branch dSph stars and foreground dwarf stars .
Additionally , this index is also capable of correctly identifying both red and blue horizontal branch stars .
We have shown that a proper cleaning of the sample results in a smaller value for the velocity dispersion of the system .
This has implications for galaxy properties derived from such velocity dispersions .