V380 Cygni Once More: A Close Detached Binary with an Evolved B‐Type Component. I. Spectroscopic Orbits

New spectroscopic and photometric observations of V380 Cygni, a bright early B-type binary with an eccentric orbit and an evolved primary, B1.5 II-III, make possible a thorough study of this important, if not unique, system. In this first of two papers, the spectroscopic material is discussed. Values of the masses and provisional values of the radii are given, as is a spectroscopic estimate of the light ratio. The determination, with some degree of reliability, of the fundamental properties of an evolved early B-type star in the 10 solar mass range enables its structure and evolutionary status to be compared with the predictions of models, including a sensitive test of convective-core overshooting. (Author)