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

Photoelectric UBVR observations of the eclipsing variable early-type star B Z Mon are presented. A new ephemeris of its minima was determined: Min I = JDH 2451 121.294(14) + 3.(d)4518062(94) x E - (172 +/- 13)(d) x 10(-10) x E -2. Brightenings by several hundredths of a magnitude, particularly pronoun ced in the red, are observed at maximum light. We solved all light curves b y 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 co ol stars are B5 V and G2 III, respectively. The cool component exhibits an ultraviolet excess of 0.'''15. Fundamental parameters of the components wer e estimated: M-h = 4.90M., M-c = 2.70M., R-h = 3.25R., and R-c = 5.20R.. Th e age of the stars is 3 x 10(5) years. The binary is a member of the OB ass ociation Mon OB2. (C) 2001 MAIK "Nauka/Interperiodica".