A new SX Phe star ( labelled SXP1 ) found from BV CCD photometry is the first to be discovered in the globular cluster M15 .
It is a blue straggler and is located 102 \arcsec . 8 north and 285 \arcsec . 6 west of the center of M15 ( 6 ) .
Mean magnitudes of SXP1 are < B > = 18 \fm 671 and < V > = 18 \fm 445 .
The amplitude of variability of SXP1 is measured to be \Delta V \approx 0.15 .
From multiple-frequency analysis based on the Fourier decomposition method , we detect two very closely separated pulsating frequencies : the primary frequency at f _ { 1 } = 24.630 c/d for both B - and V -bands , and the secondary frequency at f _ { 2 } = 24.338 c/d for the B -band and 24.343 c/d for the V -band .
This star is the second among known SX Phe stars found to pulsate with very closely separated frequencies ( f _ { 2 } / f _ { 1 } \geq 0.95 ) .
These frequencies may be explained by excitation of nonradial modes ; however , we have an incomplete understanding of this phenomenon in the case of SX Phe stars with relatively high amplitudes .
The metallicity-period and the variability amplitude-period relations for SXP1 in M15 are found to be consistent with those for SX Phe stars in other globular clusters .