OGLE Cepheids and RR Lyrae Stars in the Milky Way

We present new large samples of Galactic Cepheids and RR Lyrae stars from the OGLE Galaxy Variability Survey.Comment: 5 pages, 6 figures. Proceedings of the 22nd Los Alamos Stellar Pulsation Conference: "Wide-Field Variability Surveys: a 21st-century Perspective

An algorithm to detect blends with eclipsing binaries in planet transit searches

We present an algorithm that can detect blends of bright stars with fainter, un-associated eclipsing binaries. Such systems contaminate searches for transiting planets, in particular in crowded fields where blends are common. Spectroscopic follow-up observations on large aperture telescopes have been used to reject these blends, but the results are not always conclusive. Our approach exploits the fact that a blend with a eclipsing binary changes its shape during eclipse. We analyze original imaging data from the Optical Gravitational Lensing Experiment (OGLE), which were used to discover planet transit candidates. Adopting a technique developed in weak gravitational lensing to carefully correct for the point spread function which varies both with time and across the field, we demonstrate that ellipticities can be measured with great accuracy using an ensemble of images. Applied to OGLE-TR-3 and OGLE-TR-56, two of the planetary transit candidates, we show that both systems are blended with fainter stars, as are most other stars in the OGLE fields. Moreover, while we do not detect shape change when TR-56 undergoes transits, TR-3 exhibits a significant shape change during eclipses. We therefore conclude that TR-3 is indeed a blend with an eclipsing binary, as has been suggested from other lines of evidence. The probability that its shape change is caused by residual systematics is found to be less than 0.6%. Our technique incurs no follow-up cost and requires little human interaction. As such it could become part of the data pipeline for any planetary transit search to minimize contamination by blends. We briefly discuss its relevance for the Kepler mission and for binary star detection.Comment: Submitted to ApJ, 10 pages, 10 figure

The Araucaria Project. The Distance to the Local Group Galaxy NGC 6822 from Cepheid Variables discovered in a Wide-Field Imaging Survey

We have obtained mosaic images of NGC 6822 in V and I bands on 77 nights. From these data, we have conducted an extensive search for Cepheid variables over the entire field of the galaxy, and we have found 116 such variables with periods ranging from 1.7 to 124 days. We used the long-period ($>$ 5.6 days) Cepheids to establish the period-luminosity relations in V, I and in the reddening-independent Wesenheit index, which are all very tightly defined. Fitting the OGLE LMC slopes in the various bands to our data, we have derived distance values for NGC 6822 in V, I and ${\rm W}_{\rm I}$ which agree very well among themselves. Our adopted best distance value from the reddening-free Wesenheit index is 23.34 $\pm$ 0.04 (statistical) $\pm$ 0.05 (systematic) mag. This value agrees within the combined 1 sigma uncertainties with a previous distance value derived for NGC 6822 by McAlary et al. from near-IR photometry of 9 Cepheids, but our new value is significantly more accurate. We compare the slopes of the Cepheid PL relation in V and I as determined in the five best-observed nearby galaxies, which span a metallicity range from -1.0 to -0.3 dex, and find the data consistent with no metallicity dependence of the PL relation slope in this range. Comparing the magnitudes of 10-day Cepheids with the I-band magnitudes of the TRGB in the same set of galaxies, there is no evidence either for a significant variation of the period-luminosity zero points in V and I. The available data limit such a zero point variation to less than 0.03 mag, in the considered low-metallicity regime.Comment: Latex, Astronomical Journal accepte