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A b s t r a c t . We describe an ongoing survey to search for dark m a t t e r via lensing events of stars in the Galactic Bulge. T h e principal properties of the survey are described, a n d some preliminary results are shown for newly-discovered variables. We discuss some of the projects related to the s t u d y of the Galactic Bulge t h a t can b e addressed using these data, a n d describe the f u t u r e plans for the survey over the coming few years. K e y w o r d s : methods: observational -surveys -stars: imaging 311 //. Dejonghe and H.

The color-magnitude diagrams of ∼ 7 × 10 5 stars obtained for 12 fields across the Galactic bulge with the OGLE project reveal a well-defined population of bulge red clump giants. We find that the distributions of the apparent magnitudes of the red clump stars are systematically fainter when moving towards lower galactic l fields. The most plausible explanation of this distinct trend is that the Galactic bulge is a bar, whose nearest end lies at positive galactic longitude. We model this Galactic bar by fitting for all fields the observed luminosity functions in the red clump region of the color-magnitude diagram. We find that almost regardless of the analytical function used to describe the 3-D stars distribution of the Galactic bar, the resulting models have the major axis inclined to the line of sight by 20 − 30 deg, with axis ratios corresponding to x 0 : y 0 : z 0 = 3.5 : 1.5 : 1. This puts a strong constraint on the possible range of the Galactic bar models. Gravitational microlensing can provide us with additional constrains on the structure of the Galactic bar.

The color-magnitude diagrams of ∼ 5 × 10 5 stars obtained for 13 fields towards the Galactic bulge with the OGLE project reveal a well-defined population of bulge red clump stars. We find that the distributions of the extinction-adjusted apparent magnitudes of the red clump stars in fields lying at l = ±5 • in galactic longitude differ by 0.37 ± 0.025 mag. Assuming that the intrinsic luminosity distribution of the red clump stars is the same on both sides of the Galactic center, this implies that the distances to the red clump stars in the two fields differ by a factor of 1.185 ± 0.015. A plausible explanation of the observed difference in the luminosity distribution is that the Galactic bulge is a triaxial structure, or bar, which is inclined to the line of sight by no more than 45 • , with the part of the bar at the positive galactic longitude being closer to us. This agrees rather well with other studies indicating the presence of the bar in the center of the Galaxy. Color-magnitude diagram data are accessible over the computer network with anonymous ftp.

We present BV RI observations of the optical counterpart to the Gamma-Ray Burst (GRB) 990510 obtained with the Las Campanas 1.0-m telescope between 15 and 48 hours after the burst. The temporal analysis of the data indicates steepening decay, independent of wavelength, approaching asymptotically t −0.76±0.01 at early times (t ≪ 1 day) and t −2.40±0.02 at late times, with the break time at t 0 = 1.57 ± 0.03 days. GRB 990510 is the most rapidly fading of the well-documented GRB afterglows. It is also the first observed example of broad-band break for a GRB optical counterpart. The optical spectral energy distribution, corrected for significant Galactic reddening, is well fitted by a single power-law with ν −0.61±0.12 . However, when the B-band point is dropped from the fit, the power-law becomes ν −0.46±0.08 , indicating a possible deviation from the power-law in the spectrum, either intrinsic or due to additional extinction near the source or from an intervening galaxy at z = 1.62. Broad-band break behavior broadly similar to that observed in GRB 990510 has been predicted in some jet models of GRB afterglows, thus supporting the idea that the GRB energy is beamed, at least in some cases.

We present extensive optical and infrared photometry of the afterglow of gamma-ray burst (GRB) 030329 and its associated supernova (SN) 2003dh over the first two months after detection (2003 March 30-May 29 UT). Optical spectroscopy from a variety of telescopes is shown and, when combined with the photometry, allows an unambiguous separation between the afterglow and supernova contributions. The optical afterglow of the GRB is initially a power-law continuum but shows significant color variations during the first week that are unrelated to the presence of a supernova. The early afterglow light curve also shows deviations from the typical power-law decay. A supernova spectrum is first detectable ∼ 7 days after the burst and dominates the light after ∼ 11 days. The spectral evolution and the light curve are shown to closely resemble those of SN 1998bw, a peculiar Type Ic SN associated with GRB 980425, and the time of the supernova explosion is close to the observed time of the GRB. It is now clear that at least some GRBs arise from core-collapse SNe.

We present results of a photometric survey for variable stars in the field of the globular cluster ω Centauri. The observed region was centered roughly on the cluster core and covered 644 arcmin 2 . The cluster was monitored on 59 nights in 1999 and 2000. A total of 117 new variables were identified. Among them there are 16 RR Lyr-type stars, 35 SX Phe variables and 26 eclipsing binaries. A comprehensive catalog including all variable stars so far reported from the cluster field is presented. We list basic photometric properties and provide finding charts for a total of 392 objects. For 313 of them new BV light curves were obtained. The presented sample includes several interesting variables, such as SX Phe stars with extremely short periods of pulsation and several candidates for pulsating K giants. Optical counterparts to 9 X-ray sources detected by XMM and Chandra telescopes were identified: all of them are likely to be foreground variables not related to the cluster.

We present the first direct distance determination to a detached eclipsing binary in M33, which was found by the DIRECT Project. Located in the OB 66 association at coordinates (α, δ) = (01 : 33 : 46.17, +30 : 44 : 39.9) for J2000.0, it was one of the most suitable detached eclipsing binaries found by DIRECT for distance determination, given its apparent magnitude and orbital period. We obtained follow-up BV time series photometry, JHK s photometry and optical 1 Based on observations obtained with 1.2 meter FLWO, 2.1 meter KPNO, 3.5 meter WIYN, 8.2 meter Gemini, and 10 meter Keck-2 telescopes.