Orbit of Binary 15 Monocerotis
  In the paper new orbital elements obtained from speckle interferometric measurements only for 15 Mon are reported. For this binary they were determined earlier by combining spectroscopic and speckle measurements and spectroscopic and astrometric measurements. A new speckle measurement dating after the periastron passage of the secondary has a significant discrepancy from the ephemeridal value. Our revised orbit has a period significantly longer than the earlier ones. With regard that in the case of the earlier orbits the period was determined from spectroscopic measurements, i.e. from radial velocity data, the existing data of such kind for this binary are analyzed here and combined with our orbital elements. The first speckle measurement originates from 1988 and up to now three more speckle measurements have been made. This pair is bright (apparent magnitude of primary 4.66, i.e. 5.9 of secondary), of an early spectral type (primary O7V, secondary O9.5Vn) and is a member of open cluster NGC 2264. The distance to this cluster has been determined many times and different values have been reported. By analyzing the available data we find that the distance to 15 Mon is most likely about 750 pc, i.e. the total mass most likely about 53.4${\cal M}_{\odot}$. The new orbital elements combined with this distance yield a total mass expected for the spectral types of the components. 
Zorica Cvetkovc, Istvan Vince and Slobodan Ninkovic