  We obtained a series of more than two hundred R-band CCD images for the
crowded central (115"x77") region of the metal-poor globular cluster M15 with
an angular resolution of 0.5"-0.9" in most images. Optimal image subtraction
was used to identify variable stars. Brightness variations were found in 83
stars, 55 of which were identified with known cluster variables and the
remaining 28 are candidates for new variables. Two of them are most likely SX
Phe variables. The variability type of two more stars is incertain. The
remaining stars were tentatively classified as RR Lyrae variables. A
preliminary analysis of published data and our results shows that the
characteristics of RR Lyrae variables in the densest part (r<35") of the
cluster probably change. More specifically, the maximum of the period
distribution of first- and second-overtone (RR1, RR2) pulsating stars shifts
toward shorter periods; i.e., there is an increase in the fraction of stars
pulsating with periods < 0.3d and a deficiency of stars with 0.35d-0.40d. The
ratio of the number of these short-period RR Lyrae variables to the number of
fundamental-tone (RR0) pulsating variables changes appreciably. We found and
corrected the error of transforming the coordinates of variables V128-155 in
M15 to the coordinate system used in the catalog of variable stars in globular
clusters.
