Cluster AgeS Experiment. CCD photometry of SX phoenicis variables in the globular cluster M 55

We present CCD photometry of SX Phe variables in the held of the globular c luster M 55. We have discovered 27 variables. three of which are probable m embers of the Sagittarius dwarf galaxy. All of the SX Phe stars in M 55 lie in the blue straggler region of the cluster color-magnitude diagram. Using period ratio information we have identified the radial pulsation modes for one of the observed variables. Inspection of the period-luminosity distrib ution permits the probable identifications of the pulsation modes for most of the rest of the stars in the sample. We have determined the slope of the period-luminosity relation for SX Phe stars in hi 55 pulsating in the fund amental mode. Using this relation and the HIPPARCOS data for SX Phe itself, we have estimated the apparent distance modulus to M 55 to be (m - M)(V) = 13.86 +/- 0.25 mag.