  We present the analysis of the Blazhko effect - quasi-periodic modulation of
pulsation amplitude and/or phase - in the Galactic bulge first overtone RR
Lyrae stars (RRc). We used the data gathered during the fourth phase of the
Optical Gravitational Lensing Experiment (OGLE). Out of 10 826 analyzed RRc
stars, Blazhko effect was detected in 607 stars which constitute 5.6 percent of
the sample. It is the largest and most homogeneous sample of modulated RRc
stars analyzed so far. Modulation periods cover a wide range, from slightly
above 2 d to nearly 3000 d. Multiperiodic modulation was detected in 47 stars.
The appearance of modulation in the frequency domain was studied in detail.
Modulation manifests either as close doublets or as equidistant triplets and
multiplets centered on radial mode frequency and its harmonics. In a
significant fraction (29 percent) of stars, we have detected the modulation
frequency itself, which corresponds to the modulation of the mean stellar
brightness. Our search for period doubling effect, that was discovered recently
in modulated fundamental mode RR Lyrae stars, and triggered development of new
model behind the Blazhko modulation, yielded negative result. In 104 stars we
detected additional signals that could correspond to both radial and non-radial
modes. Statistical properties of modulated stars were analyzed in detail and
confronted with properties of non-modulated stars and of modulated fundamental
mode RR Lyrae stars. Our analysis provides constraints for the models to
explain the Blazhko phenomenon, which still remains a puzzle more than hundred
years after its discovery.
