¹ Astronomical Institute of the Charles University, V Holesovickach 2, CZ-180 00 Praha 8, Czech Republic
² Astronomical Institute of the Academy of Sciences, CZ-251 65 Ondrejov, Czech Republic

A brief history of investigations of Lyr, an emission-line binary and one of the first ever discovered Be stars is presented. A rather fast progress in our understanding of this enigmatic object during the past fifteen years is then discussed in some detail. The current picture of Lyr is that it is an eclipsing binary in a stage of mass transfer between the components. The mass-losing star is a B6-8II object, with a mass of about 3 M, which is filling the Roche lobe and sending material towards its more massive companion at a rate of about 2×10^-5 M yr^-1. This leads to the observed rapid increase of the orbital period at a rate of 19 s per year. The mass-gaining star is an early B star with a mass of about 13 M. It is completely hidden inside an opaque accretion disk, jet-like structures, perpendicular to the orbital plane and a light-scattering halo above the poles of the star. The observed radiation of the disk corresponds to an effective temperature which is much lower than what would correspond to an early B star. The disk shields the radiation of the central star in the directions along the orbital plane and redistributes it in the directions perpendicular to it. That is why the mass-losing star appears brighter of the two in the optical region of the spectrum. At present, rather reliable estimates of all basic properties of the binary and its components are available. However, in spite of great progress in understanding the system in recent years, some disagreement between the existing models and observed phase variations still remains, both for continuum and line spectrum, which deserves further effort.

Accepted by Astronomische Nachrichten
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