The First DIRECT Distance Determination 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 at coordinates (α, δ) = (01h33m46.ˢ17, + 30°44′39.″9) for J2000.0, it was one of the most suitable detached eclipsing binaries found by DIRECT for distance determination, given its apparent magnitude and orbital period. We obtained follow-up BV time-series photometry, JHKs photometry, and optical spectroscopy from which we determined the parameters of the system. It contains two O7 main-sequence stars, with masses of 33.4 ± 3.5 and 30.0 ± 3.3 M☉ and radii of 12.3 ± 0.4 and 8.8 ± 0.3 R☉, respectively. We derive temperatures of 37,000 ± 1500 and 35,600 ± 1500 K. Using BVRJHKs photometry for the flux calibration, we obtain a distance modulus of 24.92 ± 0.12 mag (964 ± 54 kpc), which is ~0.3 mag longer than the Key Project distance to M33. We discuss the implications of our result and the importance of establishing M33 as an independent rung on the cosmological distance ladder.