New short-period stellar pulsators at large Galactocentric distances

We report the discovery of 31 blue, short-period, pulsators made using data taken as part of the Rapid Temporal Survey (RATS). We find that they have periods between 51 and 83 min and full amplitudes between 0.05 and 0.65mag. Using the period-luminosity relationship for short-period pulsating stars, we determine their distance. Assuming that they are pulsating in either the fundamental or first overtone radial mode the majority are located at a distance greater than 3 kpc, with several being more than 20kpc distant. Most stars are at least 1kpc distant from the Galactic plane, with three being more than 10kpc. One is located in the direction of the Galactic anticentre and has a Galactocentric distance of ∼30kpc and is ∼20kpc below the plane: they are therefore potential tracers of the Galactic structure. We have obtained low-resolution spectra for a small number of our targets and find that they have temperatures between 7200 and 7900K and a metal content less than solar. The colours of the pulsators and the spectral fits to those stars for which we have spectra indicate that they are either SX Phe or δ Scuti stars. We estimate the number of SX Phe stars in our Galaxy and find significantly fewer per unit mass than reported in massive globular clusters or dwarf spheroidal galaxies.