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We present the first results of the analysis of 22 Blazhko stars. We find: 1) Blazhko RRab stars that are nearly pure amplitude modulators; 2) Blazhko RRab stars that have both amplitude and phase modulation; 3) A Blazhko RRab star that has an abrupt period change; 4) Proof of the Blazhko effect in RRc stars. Our data show the character of the amplitude and phase modulations of the light curves over the Blazhko cycles far better than has been previously possible.

The MACHO data base has been used to examine light curves of all red giant stars brighter than Mbol ∼ −2 in a 0.5° × 0.5° area of the LMC bar. Periods, often multiple, have been searched for in all stars found to be variable. Five distinct period-luminosity sequences have been found on the low mass (M ≲ 2.25M⊙) giant branch. Comparison of observed periods, luminosities and period ratios with theoretical models identifies Miras unambiguously as radial fundamental mode pulsators, while semi-regular variables can be pulsating in the 1st, 2nd or 3rd overtone, or even the fundamental. All these variables lie on just 3 of the 5 distinct sequences, and they all appear to be on the AGB.The fourth sequence contains red giants on the first giant branch (FGB) or at the red end of the core-helium burning loops of intermediate mass stars (M ≳ 2.25M⊙). The light curves of these stars strongly suggest that they are contact binaries, and they make up ∼0.5% of stars within 1 mag. of the FGB tip. Stars on the fifth sequence show semi-regular, eclipse-like light curves. The light curves and periods of these stars suggest that they are in semi-detached binaries, transfering mass to an invisible companion via a stellar wind or Roche lobe overflow. They make up ∼25% of AGB stars. If the existence of these red giant contact and semi-detached binaries is confirmed, then extant theories of binary star evolution will require substantial modification.

Gravitational microlensing is the most straightforward interpretation of the stellar brightenings that have been observed by our team and other experiments. These data have provided some of the most stringent limits to date on the nature of the Galaxy's dark matter halo. The number of events seen towards the LMC indicate that our Galaxy is not surrounded by a “standard” halo of MACHOs in the mass range of 10–6 to 0.3 solar masses. The observed optical depth towards the Galactic Center is an important constraint on the distribution of mass in the plane of the Galaxy.

We present the preliminary results of a frequency analysis of 1457 fundamental mode RR Lyrae (RR0) stars in the Large Magellanic Cloud (LMC) from MACHO Project photometry. We find the same classes of pulsational behavior as were found in our earlier survey of first overtone RR Lyrae (RR1) stars. Variables whose prewhitened power spectra contain one or two peaks close to the main frequency component in the original power spectra are commonly known as Blazhko-type variables. The present analysis shows the overall frequency of Blazhko-type stars in the total RR0 population analysed to date to be ≈ 10%. This is lower than the often cited Galactic field/globular rate of 20-30% (Szeidl, 1988).The incidence rate of Blazhko-type variability in the LMC appears to be about three times higher in RR0 stars than in RR1 stars. This puts important constraints on possible models of the Blazhko effect.

We present the first massive frequency analysis of the 1200 first overtone RR Lyrae stars in the Large Magellanic Cloud observed in the first 4.3 yr of the MACHO project. Besides the many new double-mode variables, we also discovered stars with closely spaced frequencies. These variables are most probably nonradial pulsators.

We have analyzed a sample of 1150 type ab, and 550 type c RR Lyrae stars found in 24 bulge fields of the MACHO database. These fields cover a range in Galactocentric distances from 0.3 to 1.6 kpc. In combination with the data on the outer bulge fields of Alard (1997) and Wesselink (1987), here we present the surface density distribution of bulge RR Lyrae between 0.3 and 3 kpc. The distribution along the line of sight of the bulge RR Lyrae population was examined on the basis of the mean magnitudes, and it was shown that the bulk of the RR Lyrae population is not barred (Alcock et al. 1998). There is a hint of a bar only in the RR Lyrae of the inner fields closer to the Galactic center. The red giant clump stars in the MACHO fields, however, clearly show a barred distribution, confirming the results of previous studies (e.g. Dwek et al. 1995, Stanek et al. 1996). In the MACHO fields studied there are about 550 clump giants per RR Lyrae star. The RR Lyrae trace metal-poor stars, which are a minor component of the bulge population. The clump giants, however, should trace the bulkmore » of the metal-rich population, foUowing underlying mass of the bulge more closely. Given the different spatial distribution, we concluded that the RR Lyrae and the clump giants trace two dif+erent populations (Alcock et al. 1998). The RR Lyrae would represent the inner extension of the Galactic halo in these fields (Minniti 1996). The observed surface distribution of RR Lyrae in the bulge fields was computed after discarding background RR Lyrae that belong to the Sgr dwarf galaxy (Alard 1996, Alcock et al. 1997). This distribution yields a power law density distribution. There is no turnover or flattening of this distribution even in the innermost fields, indicating that the RR Lyrae population is very concentrated, with core radius RC < 0.5 kpc. We also determine that the RR Lyrae surface distribution in the bulge fields is flattened, with b/a = 0.7.« less

The advantages of using near-infrared photometry of Cepheids to determine distances to nearby galaxies are now well known. In this paper we summarize the current state of the infrared period-luminosity (P-L) relations for the Galaxy, the LMC, and the SMC and present composite P-L relations derived from all available photometry. We give distance moduli for the LMC and SMC based on these data and briefly report on the status of other work in progress.

We have determined radii for Cepheids using nearinfrared K(2.2 µm) photometry and Balona’s approach to the Baade-Wesselink method of radius determination. The lower sensitivity of the K flux to temperature variations results in improved sensitivity to radius changes. We present the results of maximum-likelihood radius solutions for Cepheids with modern radial velocity curves. We point out limitations of the Balona technique as it is currently applied in the optical and suggest improvements.

The pioneering work of McGonegal et al. (1982), originally employing random-phase data for the LMC near-infrared period-luminosity (P-L) relation, is refined to mean light and extended to the SMC using new observations. We use a procedure for reducing single-phase infrared observations to mean light to obtain JHK magnitudes for Magellanic Cloud Cepheids. The new P-L relations are presented and discussed.