A study of the early-type semi-detached binary system U Herculis.

Radial velocities derived from 67 spectrograms obtained over a period of 37 yr at the DAO, Victoria, are presented. It is shown that the orbit is circular and of constant period and that the primary component is rotating synchronously. A mass ratio of 0.39±0.02 is derived and analysis of recently published light-curves using the synthesis code LIGHT confirms the semi-detached nature of the system. The final values of mass, radius and luminosity together with published uvbyβ photometry then indicate that the primary component might be expected to display β Cephei-type variability and therefore provide an explanation for the well-known asymmetries in the light-curves. The binary system belongs to a small group of short-period early-type semi-detached systems which have significantly different characteristics from classical Algol systems and are presumably the result of case A mass-exchange processes.