  We present the results of a photometric study of RR Lyrae variables in the
field of the globular cluster M55. We have discovered nine new RR Lyrae stars,
increasing the number of known variables in this cluster to 15 objects. Five of
the newly discovered variables belong to Bailey type RRc and two to type RRab.
Two background RRab stars are probable members of the Sagittarius dwarf galaxy.
Fourier decomposition of the light curves was used to derive basic properties
of the present sample of RR Lyrae variables. From an analysis of the RRc
variables we obtain a mean mass of $M=0.53\pm0.03 M_\odot$, luminosity $\log
L=1.75\pm0.01$, effective temperature $T_{eff}=7193\pm27$ K, and helium
abundance $Y=0.27\pm0.01$. Based on the $B-V$ colors, periods and metallicities
of the RRab stars we estimate the value of the color excess for M55 to be equal
to $E(B-V)=0.11\pm0.03$. Using this value we derive the colors of the blue and
red edges of the instability strip in M55. The blue edge lies at $(B-V)_0=0.20$
mag and the red edge lies at $(B-V)_0=0.38$ mag. We estimate the values of the
visual apparent and dereddened distance moduli to be $13.65\pm0.11$ and
$13.31\pm0.11$, respectively. The light curves of three of the RRc variables
exhibit changes in amplitude of over 0.1 mag on the time scale of less than a
week, rather short for the Blazhko effect, but with no evidence for another
radial pulsational frequency. However we do detect other periodicities which
are clearly visible in the light curve after removing variations with the first
overtone radial frequency. This is strong evidence for the presence of
non-radial pulsations, a behavior common for $\delta$ Scuti stars but not yet
observed among RR Lyr variables.
