THE DISCOVERY OF A NEW MASSIVE O-TYPE CLOSE BINARY - TAU-CMA (HD-57061), BASED ON HIPPARCOS AND WALRAVEN PHOTOMETRY

We present an interpretation of Hipparcos Hp and Walraven V BLUW photo metry of the brightest star in the open cluster NGC 2362 (although not necessarily associated with this cluster), HD 57061 (30 tau CMa, HR 2 782, HIP 35415). In this multiple system the central component consist of a visual double star, two O-type stars separated by 0.'' 151, whic h is also known to be a 154.(d)9 period single lined spectroscopic bin ary. it is now shown that this system also contains a massive close bi nary with a period of 1.(d)282122, most probably as the main component of the spectroscopic binary. This system therefore contains both the longest period spectroscopic binary and the shortest period eclipsing binary known for O-type stars. The shape of the light curve is charact eristic for heavily distorted double star components. An estimate has been made for various physical parameters of the system. The system se ems to be typical for some binary interaction results obtained in N-bo dy simulations for open clusters. Improved ephemeris are provided for two similar stars that were observed around the same time as HD 57061: HD 57060 and HD 167971.