Observational studies of early-type overcontact binaries: Tu Muscae

We present new spectroscopic and photometric data on the early-type overcontact binary TU Muscae. The analysis of the spectroscopic data shows that the line of sight to the system crosses three kinematically sharp and well-separated interstellar reddening sources and that the stars rotate synchronously. We present new radial velocities that are in good agreement with earlier optical velocities and, thus, do not confirm the systematically smaller velocities obtained from IUE spectra. The optical velocities are analyzed simultaneously with the photometric data to derive accurate absolute dimensions for the binary components. The results show that TU Mus consists of an O7.5 primary with M-1 = 23.5 +/- 0.8 M-. and R-1 = 7.48 +/- 0.08 R-. and an O9.5 secondary with M-2 = 15.3 +/- 0.4M(.) and R-2 = 6.15 +/- 0.07 R-. in an overcontact configuration, and that the orbital period has remained constant over the three decades covered by the observations. These results might imply that the mass transfer seen in late-type overcontact binaries does not occur in their early-type counterparts.