The First DIRECT Distance to a Detached Eclipsing Binary in M33

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, it was one of the most suitable detached eclipsing binaries found by DIRECT for distance determination, given its 4.8938 day period. We obtained follow-up BV photometry and spectroscopy from which we determined the parameters of the system. It contains two O7 main sequence stars with masses of 33.4 ± 3.5 M ⊙ and 30.0 ± 3.3 M ⊙ and radii of 12.3 ± 0.4 R ⊙ and 8.8 ± 0.3 R ⊙, respectively. We derive temperatures of 37000 ± 1500 K and 36000 ± 1500 K and determine the reddening E(B − V) = 0.14 ± 0.03. Using HST photometry for flux calibration in the V band, we obtain a preliminary distance modulus of 24.87 ± 0.16 mag (942 ± 73 kpc). The photometry and thus distance is subject to revision in the final paper.