  Results of a search for variable stars in the central region of the globular
cluster M13 are presented. Prior to this study, 36 variable and suspected
variable stars were known in this cluster (Osborn 2000; Clement et al. 2001).
Of these stars, five were not observed by us. We find v3, v4, v10, v12, and v13
to be constant in light. Surprisingly, only two out of the ten variable star
candidates of Kadla et al. (1980) appear to be variable. Both are RRc
variables. Additionally, three RR Lyrae stars and one SX Phoenicis variable are
discovered. Three close frequencies are detected for an RRc star v36. It
appears that this variable is another multi-periodic RR Lyrae star pulsating in
non-radial modes. Light curves of the three known BL Herculis stars and all
known RR Lyrae stars are presented. The total number of known RR Lyrae stars in
M13 is now nine. Only one is an RRab star. The mean period of RRc variables
amounts to 0.36 +/- 0.05 d, suggesting that M13 should be included in the group
of Oosterhoff type II globular clusters. Mean V magnitudes and ranges of
variation are derived for seven RR Lyrae and three BL Herculis variables.
Almost all observed bright giants show some degree of variability. In
particular, we confirm the variability of two red giants announced to be
variable by Osborn (2000) and in addition find five new cases.
