  We analyse the OGLE-IV photometry of the first overtone and double-mode RR
Lyrae stars (RRc/RRd) in the two fields towards the Galactic bulge observed
with high cadence. In 27 per cent of RRc stars we find additional non-radial
mode, with characteristic period ratio, P x /P 1O \in (0.6, 0.64). It strongly
corroborates the conclusion arising from the analysis of space photometry of
RRc stars, that this form of pulsation must be common. In the Petersen diagram
the stars form three sequences. In 20 stars we find two or three close
secondary modes simultaneously. The additional modes are clearly
non-stationary. Their amplitude and/or phase vary in time. As a result, the
patterns observed in the frequency spectra of these stars may be very complex.
In some stars the additional modes split into doublets, triplets or appear as a
more complex bands of increased power. Subharmonics of additional modes are
detected in 20 per cent of stars. They also display a complex structure.
  Including our previous study of the OGLE-III Galactic bulge data, we have
discovered 260 RRc and 2 RRd stars with the additional non-radial mode, which
is the largest sample of these stars so far. The additional mode is also
detected in two Blazhko RRc stars, which shows that the modulation and
additional non-radial mode are not exclusive.
