  Non-radial modes are excited in classical pulsators, both in Cepheids and in
RR Lyrae stars. Firm evidence come from the first overtone pulsators, in which
additional shorter period mode is detected with characteristic period ratio
falling in between 0.60 and 0.65. In the case of first overtone Cepheids three
separate sequences populated by nearly 200 stars are formed in the Petersen
diagram, i.e. the diagram of period ratio versus longer period. In the case of
first overtone RR Lyrae stars (RRc stars) situation is less clear. A dozen or
so such stars are known which form a clump in the Petersen diagram without any
obvious structure.
  Interestingly, all first overtone RR Lyrae stars for which precise
space-borne photometry is available show the additional mode, which suggests
that its excitation is common. Motivated by these results we searched for
non-radial modes in the OGLE-III photometry of RRc stars from the Galactic
bulge. We report the discovery of 147 stars, members of a new group of
double-mode, radial-non-radial mode pulsators. They form a clear and tight
sequence in the Petersen diagram, with period ratios clustering around 0.613
with a signature of possible second sequence with higher period ratio (0.631).
The scatter in period ratios of the already known stars is explained as due to
population effects. Judging from the results of space observations this still
mysterious form of pulsation must be common among RRc stars and with our
analysis of the OGLE data we just touch the tip of the iceberg.
