We describe an ongoing survey to search for dark matter via lensing events of stars in the Galactic Bulge. The principal properties of the survey are described, and some preliminary results are shown for newly-discovered variables. We discuss some of the projects related to the study of the Galactic Bulge that can be addressed using these data, and describe the future plans for the survey over the coming few years.

From analysis of a photometrically selected sample of 175 metal-poor field stars with main sequence gravities (hereafter BMPs) and UBV colors blueward of the most metal-poor globular cluster main-sequence turnoffs, 0.15<(B - V)0<0.35, and properties of the candidate lists of the HK objective prism survey, we calculate the space density and a suitably defined specific frequency of BMPs within approximately 2 kpc of the Sun. We consider two cases. If we adopt the luminosities and color distribution of globular cluster blue stragglers (hereafter BS) for BMPs, the BMP space density is approximately 350 kpc-3, from which we obtain a specific frequency S(BMP)-8, a value 9 times larger than that of BSs in globular clusters. From this result, we conclude that cluster-type BSs are but a minor component of the field BMPs and that the remainder must be of a different nature. If we adopt luminosities of metal-poor models halfway between the Zero Age Main Sequence and the phase of core hydrogen exhaustion, we obtain a space density of 450 (+300,-150) kpc-3 and a specific frequency S(BMP) = 10(+5,-3). From a subsample of 107 BMPs with available radial velocities we derive a galactic rotation of 128 km s-1 and an isotropic velocity dispersion of sigma(rphitheta) approximately 90 km s-1 values intermediate between those of halo and thick-disk populations. From analysis of a larger sample of stars on 0. 15<(B - V)0<0.35 binned by a crude line-blanketing parameter, we find that our results are insensitive to adopted BMP selection criteria: none of these subsamples of A- and early F-type stars above the galactic plane possess disk kinematics. The region of the UBV two-color diagram occupied by BMPs could be populated by metal-deficient, main-sequence gravity stars with ages substantially younger than those of the metal-poor halo. Because we cannot imagine how or where the observed local population of BMPs could have been produced within our galaxy during the past 3 to 10 Gyr, we suggest that BMPs are the bluest members of metal-poor intermediate-age main sequences accreted, probably, from dwarf spheroidal satellites of the Milky Way during the past 10 Gyr. We discuss observational consequences of this suggestion.

The analysis of the first three years of the OGLE data revealed 12 microlensing events of the Galactic bulge stars, with the characteristic time scales in the range 8.6 < t(0) < 80 days, where t(0) = R(E)/V. A complete sample of nine events gave the optical depth to gravitational microlensing larger than (3.3 +/- 1.2) x 10(-6), in excess of current theoretical estimates, indicating a much higher efficiency for microlensing by either bulge or disk lenses. The lenses are likely to be ordinary stars in the Galactic bar, which has its long axis elongated towards us. At this time we have no evidence that the OGLE events are related to dark matter. The OGLE color magnitude diagrams reveal the presence of the Galactic bar and a low density inner disk region similar to 4 kpc in radius. A catalogue of a few thousand variable stars is in preparation.

The color temperature and Balmer jump inferred from UBV and Stromgren photometric indices, the low gravity and low metallicity derived from spectrum analysis, and weakness of all CH molecular lines combine to suggest that HD 195636 is in an evolutionary state near the transition between the horizontal branch and asymptotic giant branch. The projected equatorial rotational velocity of PID 195636 is V-e sin i=25 km/s, a value at least 2.5 times greater than that expected if known blue horizontal branch axial rotators in globular clusters conserve envelope angular momentum during horizontal branch evolution. Constancy of the radial velocity on time scales of 3, 10, and 90 days indicates that the axial rotation is not due to co-rotation in a short-period binary. Therefore, it seems most plausible that transfer of angular momentum from core to envelope occurred during HB evolution. The Balmer line profiles are peculiar. H alpha is abnormally shallow, as if the core were filled by emission, and higher members of the Balmer series are significantly broader than those of HD 200564, a slightly cooler giant star of similar metallicity. The space velocity of HD 195636 calculated for an assumed RHB/AGB luminosity, M-V=-0.5, is 470 km/s retrograde, a high but not extraordinary value. (C) 1997 American Astronomical Society.

CS 22966-043 is an ultra-short-period pulsating star with high velocity (RV = -266 km s(-1)) discovered during the course of a search for spectroscopic binaries among blue metal-poor field stars, in progress since 1992. With respect to period (0.0374 days), mean color ((B-V) = 0.24), and metal abundance ([Fe/H] approximate to -2.4), it closely resembles the SX Phoenicis stars found among the blue stragglers in NGC 5053. CS 22966-043 also is the primary of a spectroscopic binary with (probable) period of 430 days. Light-travel time across the projected orbit, as large as 0.0037 days, must be added to the times of observation to combine data obtained in different years with minimal phase dispersion. If CS 22966-043 is, indeed, a blue straggler formed by binary interaction as is now generally believed, then it seems most probable that the interaction was one of mass transfer from the present-day secondary during its post-main-sequence evolution rather than merger of a close binary. The latter option would require that this rare field star was, in addition, a member of a primordial triple system.

The blue metal-poor (BMP) star CS 22966-043 is an SX Phoenicis star and the primary of a spectroscopic binary with a provisional orbital period of similar to 430 days. Radial velocity and UBV photometric observations of this star made in 1998 require downward revision of the orbital period to 319 days. The BMP star CS 29499-057 also appears to be an SX Phoenicis star with small amplitude (Delta V similar to 0.04 mag) and short period (P = 0.0417 days), on the basis of photometric and radial velocity observations obtained in 1998. There is some indication that it too may be the primary of a spectroscopic binary. Three other BMP stars have radial velocity standard deviations greater than those of 17 BMP radial velocity standards. We suggest that they may be small-amplitude SX Phoenicis stars. Finally, the BMP star CS 29497-017 is shown to be a short-period velocity variable (P = 0.302 days) on the basis of observations accumulated over an interval of 2200 days, but we were unable to detect a light variation in 1998 July. Therefore, the nature of the velocity variation of this object remains uncertain.

As we enter the third millennium, I reflect on what has been accomplished by the search for extremely metal poor stars during the past 30 years, what might be done better, and what the future holds for this subject.