Toward an Orbit for the High‐Luminosity Cepheid T Monocerotis

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Abstract
We have obtained new velocities of the long-period Cepheid T Mon from the ground and velocities of its hot companion with the Hubble Space Telescope (HST) and the International Ultraviolet Explorer (IUE). Although observations do not cover a full orbit, both the maximum and minimum orbital velocities have now been obtained. We present a preliminary orbit and discuss the uncertainties in the orbital parameters. The velocities for the companion appear to be inconsistent with binary orbital motion, and it is likely that the companion is itself a binary in a short-period orbit. The HST spectrum of the companion shows that it is a chemically peculiar star, probably magnetic. Because it is coupled with the more massive Cepheid, it must be very close to the zero-age main sequence. The well-determined mass function from the preliminary orbit implies that the inclination of the long-period system is close to 90°.

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