V578 Mon is a system of two early B-type stars in the Rosette Nebula
star-forming region (NGC 2244), and is one of only nine eclipsing binaries with
component masses greater than 10 M\odot whose physical parameters have been
determined with an accuracy of better than 3%. It is therefore a benchmark
system for evolutionary and stellar structure models of newly formed massive
stars. Combining our multi-band light curves spanning 40 yr with previous light
curve data from the literature, we fit a model light curve that for the first
time includes the effects of apsidal motion of the system. We measure an
apsidal period of 33.48+0.10-0.06 yr. As a consequence of incorporating the
apsidal motion into the modeling of the system's orbital parameters, we
determine an updated eccentricity of e = 0.07755+0.00022-0.00027, which differs
significantly from the value previously reported in the literature. Evidently,
the inclusion of apsidal motion in the light curve modeling significantly
affects the eccentricity determination. Incorporating these key parameters into
a comprehensive model of the system's physical parameters-including internal
structure constraints- will bring V578 Mon to the next level of benchmark
precision and utility.Comment: 17 pages, 7 figures, to appear in the Astronomical Journa