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

¹ Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warszawa, Poland
² Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101, USA
³ Astronomical Observatory of Adam Mickiewicz University, ul. Sloneczna 36, 60-286 Poznan, Poland

Corresponding author: W. Pych,pych@sirius.astrouw.edu.pl

Received: 28 June 2000
Accepted: 27 November 2000

Abstract

We present CCD photometry of SX Phe variables in the field of the globular cluster M 55. We have discovered 27 variables, three of which are probable members 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 distribution 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 M 55 pulsating in the fundamental mode. Using this relation and the HIPPARCOS data for SX Phe itself, we have estimated the apparent distance modulus to M 55 to be mag.