BINARY STAR ORBITS FROM SPECKLE INTERFEROMETRY .5. A COMBINED SPECKLESPECTROSCOPIC STUDY OF THE O-STAR BINARY-15 MONOCEROTIS

We report on the discovery of a speckle binary companion to the O7 V ( f) star 15 Monocerotis. A study of published radial velocities in conj unction with new measurements from KPNO and IUE suggests that the star is also a spectroscopic binary with a period of 25 years and a large eccentricity. Thus, 15 Mon is the first O star to bridge the gap betwe en the spectroscopic and visual separation regimes. We have used the s tar's membership in the cluster NGC 2264 together with the cluster dis tance to derive masses of 34 and 19 M. for the primary and secondary, respectively. Several of the He I line profiles display a broad shallo w component which we associate with the secondary, and we estimate the secondary's classification to be O9.5 Vn. The new orbit leads to seve ral important predictions that can be tested over the next few years.