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 _ { 1 O } \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 .