ИСТИНА

We report the results of CCD $BVRc$ photometry and medium resolution spectroscopy of OB star No.0712 in the galaxy M 33 [Ivanov et al. 1993] identified with the emission line star Ha19 [Fabrika, Sholukhova 1995]. The astrometric position of the star is 1h33m39s.468 +30d45'40''.25, Eq. 2000.0. The first photometric and spectroscopic investigations of this star were described in the paper of [Sharov, Goranskij, Fabrika (1997)]. It is an eclipsing binary with the orbital period of 33.108 day. Ha19 is associated with an H II region having two-lobe structure which is seen in two images taken by [Courtes et al. (1993)] using the Russian 6-m telescope BTA. The blue central star and two red lobes of this H II region are seen in the high-resolution color image of M33 central part taken by [King et al. (2001)] with KPNO 4 m Mayall Telescope. The overall size of H II region is 30 pc. The star is not associated with any X-ray or radio source. In 2001 - 2009, we performed CCD BVRC photometry with SAO 1 m reflector and CCD photometer based on the chip EEV 42-40, and SAI Crimean station 60 cm telescope with CCD VersArray manufactured by Princeton Instruments. Light curves, B-V and V-RC color curves are shown in Fig. 1a and 1b. The range of variability was 16.90 - 17.55 mag in the $V$ band, with the magnitude of Min II at 17.35 mag. We determined light elements Min I = 2453593.53 + 33d.108(+/-0d.003)xE. This formula confirms the results of photographic observations by [Sharov, Goranskij, Fabrika (1997)]. Color indices are the following: B-V=+0.08 mag, V-R_C=+0.16 mag. With these indices, Ha19 is located near the red edge of OB stars' main sequence, 1.5 mag lower than the brightest O supergiant in the CM diagram by [Massey et al. (2006)] of M33 galaxy. The dominating feature of light curves is a hump just before the primary eclipse due to bright extended spot on the surface of a hot luminous companion. This spot is as bright as 25000 Suns. We assume that Ha19 is a merging binary which has a large rate of mass exchange, and therefore the radiative heat transfer has been changed partly by circulation in the volume of the hot gainer. This circulation transports very hot matter from the deep layers of the star envelope to the surface.