THE BINARY-SYSTEM CONTAINING THE CLASSICAL CEPHEID-T MON

Several new results are presented for the binary system containing the 27d classical Cepheid T Mon. New radial velocities for the Cepheid ha ve been obtained, which confirm the decreasing orbital motion at the c urrent epoch. The spectral type of the companion (B9.8 V) has been det ermined from an IUE low resolution spectrum. An IUE high resolution sp ectrum has been measured to search for the velocity of the companion. A velocity signal at +36 km s-1 on JD 2,446,105.21 has been tentativel y identified as the velocity of the companion, but confirmation of thi s velocity would be very valuable. Results based on this tentative ide ntification of the velocity are that the companion does not have a hig h projected rotation velocity, that the companion is unlikely to be a short period binary, and that the gamma velocity of the system is betw een 20 and 36 km s-1. The luminosity and temperature of both the Cephe id and the companion are well determined from the satellite and ground -based observations and the Cepheid PLC relation. However, the compani on is above the ZAMS in the H-R diagram, which is inconsistent with th e large luminosity difference between the two stars. High rotation for the companion (viewed pole-on) is a possible explanation. The lower l imit to the mass function (from the lower limits to the orbital period and amplitude) requires a very high eccentricity for the system for r easonable estimates for the masses of the two stars.