  We report the discovery of an RR Lyrae star that experienced a switching of
its pulsation mode. OGLE-BLG-RRLYR-12245 was discovered as a double-mode RRd
star from the observations conducted in years 2001-2006 during the third phase
of the Optical Gravitational Lensing Experiment (OGLE-III). The OGLE-IV
observations carried out since 2010 reveal that this object is now a
fundamental-mode RRab star, with no sign of the first-overtone pulsation. The
analysis of the OGLE photometry shows that the final stage of the mode
switching occurred on a relatively short timescale of a few months in 2005. We
study the behavior of the star during this process, showing changes of the
pulsational amplitudes and periods. We also discuss possible causes for the
mode switching in RR Lyr stars.
