OGLE small-amplitude variables in the Galactic bar

Among over 200 000 Galactic bulge variable stars in the public domain Optical Gravitational Lensing Experiment (OGLE) catalogue, we found over 15 000 red giant variables following two well-defined period–amplitude relations. The periods are in the range 10 < P < 100 d, and amplitudes in the range 0.005 < A < 0.13 mag in the I band. The variables cover a broad range of reddening-corrected colours, 1 < (V−I)0 < 5, and a fairly narrow range of extinction-corrected apparent magnitudes, 10.5 < I0 < 13. A subset of variables (type A) has an rms scatter of only 0.44 mag. The average magnitudes for these stars are well correlated with the Galactic longitude, and vary from Ik,0= 11.82 for l=+ 8° to Ik,0= 12.07 for l=−5°, clearly indicating that they are located in the Galactic bar. Most variables have several oscillation periods.