Light curve solution of HD 93205 (O3 V+O8V)containing the earliest known star in a well-studied binary

We present the results of an extensive photometric study of the O3 V + O8 V binary HD 93205 (P similar or equal to 6 degrees 08, e = 0.46). The primary O3 V star has by far the earliest known spectral type of a normal star in a cataloged close binary. Some 186 individual differential observations, each of precision similar to 0.003 mag, were obtained over a contiguous interval of similar to 3 months in a narrow, visual-continuum bandpass. The amplitude of photometric variability is very low, about 0.02 mag, with most of the light changes occurring near periastron passage. Analysis of the light variations with a state-of-the-art binary model in an eccentric orbit leads to the conclusion that the system does not exhibit eclipses. Rather, the light variations are due mainly to orbital revolution of tidally distorted stars. However, there is an additional very small, but real, systematic decreasing trend in the light curve of the system approximately centered on the apastron passage, i.e., between orbital phases 0.35 and 1.0, which cannot be accounted for with present models. A nonuniform brightness distribution on the surface of the star(s), whose origin remains a mystery, may be responsible for this effect. Another plausible explanation of the trend may be related to turbulent viscosity, causing tidal lag. Despite this problem, one can estimate the range of possible values for the orbital inclination angle, e.g., at the 5% significance level, 75 degrees greater than or equal to i greater than or equal to 35 degrees, which leads to the masses M-O3 similar or equal to 32-154 M. and M-O8 similar or equal to 14-68 M.. The best-fit value, i = 60 degrees, yields M-O3 = 45 M. and M-O8 = 20 M.. The latter value is compatible with the reliable masses of the two O8 V stars (22 M.) in the detached eclipsing binary system EM Car. This would imply that at least one of the earliest known main-sequence O3 stars has relatively modest mass, compared to evolutionary masses of the most massive stars, which are claimed elsewhere in the literature to reach up to at least 100 M..