Letter | Published:

Cygnus X-1—a Spectroscopic Binary with a Heavy Companion ?

Abstract

WE have reported1 that the spectrum, colours and interstellar features in the star HD 226868, which is coincident with the X-ray star Cygnus X-1 and a radio star2,3, were those of a normal BOIb supergiant. We made measurements of the radial velocity of the star from August 1971 to October 1971 and find it to be a velocity variable, whose changes of velocity correlate with changes in the X-ray flux.

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