  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.
