We present new B , V and I CCD time-series photometry for 177 variable stars in a 13 \arcmin \times 13 \arcmin field centered on the globular cluster M54 ( lying at the center of the Sagittarius dwarf spheroidal galaxy ) , 94 of which are newly identified variables .
The total sample is composed of 2 anomalous Cepheids , 144 RR Lyrae stars ( 108 RR0 and 36 RR1 ) , 3 SX Phoenicis , 7 eclipsing binaries ( 5 W UMA and 2 Algol binaries ) , 3 variables of uncertain classification and 18 long-period variables .
The large majority of the RR Lyrae variables likely belong to M54 .
Ephemerides are provided for all the observed short-period variables .
The pulsational properties of the M54 RR Lyrae variables are close to those of Oosterhoff I clusters , but a significant number of long-period ab type RR Lyrae are present .
We use the observed properties of the RR Lyrae to estimate the reddening and the distance modulus of M54 , E ( B-V ) =0.16 \pm 0.02 and ( m-M ) _ { 0 } =17.13 \pm 0.11 , respectively , in excellent agreement with the most recent estimates .
The metallicity has been estimated for a subset of 47 RR Lyrae stars , with especially good quality light curves , from the Fourier parameters of the V light curve .
The derived metallicity distribution has a symmetric bell shape , with a mean of \langle [ Fe / H ] \rangle = -1.65 and a standard deviation \sigma = 0.16 dex .
Seven stars have been identified as likely belonging to the Sagittarius galaxy , based on their too high or too low metallicity .
This evidence , if confirmed , might suggest that old stars in this galaxy span a wide range of metallicities .