The discovery of a new massive O-type close binary: τ CMa (HD57061), based onHipparcos andWalraven photometry

We present an interpretation of Hipparcos Hp and WalravenV BLUW photometry of the brightest star in the open clusterNGC 2362 (although not necessarily associatedwith this cluster), HD 57061 (30τ CMa, HR 2782, HIP 35415). In this multiple system the central component consist of a visual double star, two O-type stars separated by 0. ′′151, which is also known to be a 154. d9 period single lined spectroscopic binary. It is now shown that this system also contains a massive close binary with a period of 1. d282122, 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 characteristic for heavily distorted double star components. An estimate has been made for various physical parameters of the system. The system seems to be typical for some binary interaction results obtained in N-body 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.