Be star
The spectrum of HR 6819 contains both narrow and broadened lines. The broad lines originate from the rapidly-rotating Be star, while the narrow lines are from a more slowly-rotating B-class star. The radial velocity variations of the lines indicate that the normal B-class star is in a 40-day orbit, but the Be star does not show detectable oscillation. Assuming that these two stars have similar masses, this suggests that there is a third, invisible, body in the system, as the other component in the 40-day orbit. Analysis of the orbital parameters suggests that the third body is so massive that it can only be a black hole.