Fundamental Properties And Distance To The LMC Eclipsing Binary Macho* J053648.7-691700F. Maloney, T. Mizusawa, E. Fitzpatrick, A. Prsa, E. GuinanVillanova University ABSTRACT We present an analysis and preliminary fundamental properties of the eccentric eclipsing and spectroscopic binary system, MACHO*053648.7–691700, in the Large Magellanic Cloud (LMC). This is a detached system consisting of two similar late-O or early-B components with a 3.85 day period. Previously published CCD u, Vj, andIc photometry (Bayne et al. 2004) and HST/STIS spectroscopy obtained by us were used to analyze the system. The modeling of the light curve reveals the temperature ratio and relative radii of the component stars, as well as the orbital inclination, longitude of periastron, and eccentricity. The radial velocity study, when combined with the photometry, reveals the temperatures, radii, reddening, and masses of the stars. Of particular importance is the distance to this system, obtained from a knowledge of the radii, temperatures, and reddening, since the location of the system is within ~7 arc-min of SN 1987A, whose distance is thought to be known accurately. METHODS & OBSERVATIONS RESULTS CONCLUSIONS ORBITAL AND STELLAR PROPERTIES Period = 3.853529 ± 0.000005 d Inclination = 84.5± 0.25 deg Eccentricity = 0.185 ± 0.005 = 25 ± 0.5 deg Mass = 17 ± 3 Msun R1 = 6.7 ± 0.5 Rsun R2 = 6.3 ± 0.5 Rsun T1 = 30 000 K adopted T2 = 29 000 K Using the 1-meter Cassegrain telescope at the Mount John observatory, Bayne et al. obtained CCD photometry of the target system in u, Vj, and Ic. From these observations, taken between April 1999 and July 2002, they were able to determine that the system is detached, and they obtained values for the system’s ephemeris, eccentricity, and apsidal motion. However, without spectroscopy, definitive values for the mass ratio, size of the orbit, and stellar radii cannot be obtained. Therefore, as part of a larger program to study binary systems in the LMC, we obtained STIS spectroscopy using the Hubble Space Telescope. Ten spectra were taken between November 2000 and February 2001. One of the spectra is shown below. Our preliminary results for the target system agree favorably with those presented by Bayne et al. The orbit is elliptical, and shows evidence of apsidal motion. The component stars are detached, and similar in mass, size, and temperature. A complete analysis of this system requires the use of spectral disentangling to provide more accurate radial velocities, and the use of a Wilson-Devinney engine to simultaneously fit the photometry and spectroscopy. With these definitive parameters for the components, we can determine an accurate distance to SN 1987A. REFERENCES Alcock et al., 1997, AJ…114…326A Bayne et al., 1994, MNRAS 349, 833-840 Bayne et al., 2004, RevMexAA (Servie de Conferencias), 21, 116-120 The prominent Si III and C II absorption features were used to measure Doppler shifts and to compute radial velocities. IRAF and IDL routines were employed to fit gaussians to the lines, as well as to fit portions of a 30000 K early B star model atmosphere. Using the published photometry and our radial velocities, we used Binary Maker to model the system.