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 \pm 0.03 M _ { \odot } , luminosity \log L = 1.75 \pm 0.01 , effective temperature T _ { eff } = 7193 \pm 27 K , and helium abundance Y = 0.27 \pm 0.01 .
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 \pm 0.03 .
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 \pm 0.11 and 13.31 \pm 0.11 , 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 .