Double-periodic blue variables in the Magellanic Clouds

We report the discovery, based on an inspection of the OGLE-II database, of a group of blue variables in the Magellanic Clouds showing simultaneously two kinds of photometric variability: a short-term cyclic variability with typical amplitude $\Delta I \sim$ 0.05 mag and period $P_{1}$ between 4 and 16 days and a sinusoidal, long-term cyclic oscillation with much larger amplitude $\Delta I \sim$ 0.2 mag with period $P_{2}$ in the range of 150-1000 days. We find that both periods seems to be coupled through the relationship $P_{2}$ = 35.2 $\pm$ 0.8 $P_{1}$. In general, the short term variability is reminiscent of those shown by Algol-type binaries. We propose that the long-term oscillation could arise in the precession of a elliptical disc fed by a Roche-lobe filling companion in a low mass ratio Algol system.Comment: submitted to A&A letter

Eclipsing Binaries in the Young LMC Cluster NGC 1850

I present light curves for two detached eclipsing binary stars in the region of the LMC cluster NGC 1850, which is possibly a young globular cluster still in formation. One, a likely spectral type O star, is a newly detected eclipsing binary in the region of the very young subcluster NGC 1850A. This binary is among a small number of highly massive O-type stars in binary systems found in LMC clusters. These two eclipsing binaries are the first discovered in the well studied NGC 1850, and the O-type star is the first eclisping binary found in NGC 1850A. Light curves for two NGC 1850 region Cepheid variables are also shown. Discovering two eclipsing binaries in the young globlular-like cluster NGC 1850 is discussed in terms of the importance of the binary fraction to globular cluster evolution.Comment: 27 pages, 11 figures; Accepted for publication in December 2004 PASP. (No changes in paper from original submitted version; only this comment reflecting acceptance by the Proceedings of the Astronomical Society of the Pacific has been added.

Seismology of triple-mode classical Cepheids of the Large Magellanic Cloud

We interpret the three periods detected in OGLE LMC Cepheids SC3-360128 and SC5-338399 as corresponding to the first three overtones of radial pulsations. This interpretation imposes stringent constraints on parameters of the stars and on their evolutionary status, which could only be the first crossing of the instability strip. Evolutionary models reproducing measured periods exist only in a restricted range of metallicities (Z=0.004-0.007). The models impose an upper limit on the extent of overshooting from the convective core. Absolute magnitude of each star is confined to a narrow interval. This allows to derive a new estimate of the distance to the LMC. We obtain m-M ranging from 18.34mag to 18.53mag, with a systematic difference between the two stars of about 0.13mag. The rates of period change predicted by the models are formally in conflict with the derived observational limits, though the uncertainities of measured dP/dt may be underestimated. If the discrepancy is confirmed, it would constitute a significant challenge to the stellar evolution theory.Comment: 9 pages, 2 figures, accepted for publication in A&

OGLE-2003-BLG-238: Microlensing Mass Estimate of an Isolated Star

Microlensing is the only known direct method to measure the masses of stars that lack visible companions. In terms of microlensing observables, the mass is given by M=(c^2/4G)\tilde r_E \theta_E and so requires the measurement of both the angular Einstein radius, \theta_E, and the projected Einstein radius, \tilde r_E. Simultaneous measurement of these two parameters is extremely rare. Here we analyze OGLE-2003-BLG-238, a spectacularly bright (I_min=10.3), high-magnification (A_max = 170) microlensing event. Pronounced finite source effects permit a measurement of \theta_E = 650 uas. Although the timescale of the event is only t_E = 38 days, one can still obtain weak constraints on the microlens parallax: 4.4 AU < \tilde r_E < 18 AU at the 1 \sigma level. Together these two parameter measurements yield a range for the lens mass of 0.36 M_sun < M < 1.48 M_sun. As was the case for MACHO-LMC-5, the only other single star (apart from the Sun) whose mass has been determined from its gravitational effects, this estimate is rather crude. It does, however, demonstrate the viability of the technique. We also discuss future prospects for single-lens mass measurements