As the RR Lyrae stars evolve on the Hertzsprung-Russell diagram they are believed to become short period Type II Cepheids , known as BL Her type ( with a pulsation period from 1 to 3 - 8 days ) .
Assuming that their mass is around 0.5 - 0.6 { M } _ { \odot } , and that they are low metallicity objects , they were thought to belong to the halo of the Milky Way .
We investigated seven Galactic short period Type II Cepheids ( BL Her , SW Tau , V553 Cen , DQ And , BD Cas , V383 Cyg , and KT Com ) in order to establish their membership within the Galactic structure using the kinematic approach . Gaia should provide us with more data needed to conduct the study of the whole sample .