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We have analyzed a sample of 27,258 fundamental-mode RR Lyrae variable stars(type RRab) detected recently toward the Galactic bulge by the OpticalGravitational Lensing Experiment (OGLE) survey. The data support our earlierclaim that these metal-poor stars trace closely the barred structure formed ofintermediate-age red clump giants. The distance to the Galactic center (GC)inferred from the bulge RR Lyrae stars is R_0=8.27+/-0.01(stat)+/-0.40(sys)kpc. We show that their spatial distribution has the shape of a triaxialellipsoid with an major axis located in the Galactic plane and inclined at anangle of i=20+/-3 deg to the Sun-GC line of sight. The obtained scale-lengthratio of the major axis to the minor axis in the Galactic plane and to the axisvertical to the plane is 1:0.49(2):0.39(2). We do not see the evidence for thebulge RR Lyrae stars forming an X-shaped structure. Based on the light curveparameters, we derive metallicities of the RRab variables and show that thereis a very mild but statistically significant radial metallicity gradient. About60% of the bulge RRab stars form two very close sequences on theperiod-amplitude (or Bailey) diagram, which we interpret as two major old bulgepopulations: A and B. Their metallicities likely differ. Population A is aboutfour times less abundant than the slightly more metal-poor population B. Mostof the remaining stars seem to represent other, even more metal-poorpopulations of the bulge. The presence of multiple old populations indicatesthat the Milky Way bulge was initially formed through mergers.

Deciphering the 3D structure of the old Galactic bulge from the OGLE RR Lyrae stars