Period-luminosity relationship for SX Phoenicis stars in Galactic globular clusters

We compiled a list of about 250 SX Phoenicis stars known in Galactic globular clusters in order to study period-luminosity relation for this type of variable. The absolute magnitudes of these stars are derived using metallicity-luminosity calibration for RR Lyrae stars. The mixture of different radial and non-radial modes present in SX Phoenicis stars and the lack of unique method of mode identification cause the difficulties in defining strict period-luminosity relation. As a solution we propose to use confirmed double-mode radial pulsators.