Close binary systems in star-forming regions: BZ Mon in the OB association Mon OB2

Photoelectric UBVR observations of the eclipsing variable early-type star BZ Mon are presented. A new ephemeris of its minima was determined: $$Min I = JDH 2451 121.294(14) + 3\mathop .\limits^d 4518062(94) \times E - (172 \pm 13)^d \times 10^{ - 10} \times E^2$$ . Brightenings by several hundredths of a magnitude, particularly pronounced in the red, are observed at maximum light. We solved all light curves by Lavrov’s method and obtained highly accurate photometric orbital elements of the system. The positions of its components are analyzed in two-color (U-B), (B-V) and (U-B), (V-R) diagrams. The spectral types of the hot and cool stars are B5 V and G2 III, respectively. The cool component exhibits an ultraviolet excess of $$0\mathop .\limits^m 15$$ . Fundamental parameters of the components were estimated: M h = 4.90M ⊙, M c = 2.70M ⊙, R h = 3.25R ⊙, and R c = 5.20R ⊙. The age of the stars is 3×105 years. The binary is a member of the OB association Mon OB2.

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