RR Lyrae stars with variable mean magnitudes

Context. A number of RR Lyrae stars exhibit variable mean magnitudes in the OGLE survey light curves of the Galactic bulge. Hitherto, this phenomenon has not been studied, as it was generally assumed to be related to photometric issues. Aims. We investigate whether the mean magnitude variability of RR Lyrae variables is due to genuine astrophysical phenomena. Methods. We used the extended, and in many cases overlapping, light curves from multiple microlensing surveys to study RR Lyrae stars with apparent mean-magnitude variations. A modified Fourier-series based fitting method is introduced to analyze the light curves showing mean-magnitude variations. Data from infrared surveys were also used to construct spectral energy distributions (SEDs). Results. Presented here are 72 stars where the mean-magnitude variations are most probably of genuine astrophysical origin, rather than the result of photometric problems. The ratio of variation between the V and I bands is compatible with variable extinction by dust in most cases, but no infrared excess is detected in the SEDs. The occurrence rate of the phenomenon, after correcting for selection effects, is ∼0.9% among RR Lyrae variables in the OGLE bulge fields.