  The X-ray binary system 4U 2206+54 hides many mysteries. Among them, the
surprising behavior of both of its components: the O9.5 dwarf star
BD+53$^\circ$2790 and a slowly rotating neutron star. BD+53$^\circ$2790 misled
the astronomers showing itself very likely as a Be star. However, a deeper
spectral analysis and more intense monitoring, revealed that the real picture
was a bit more complicated: a) Although it shows evidence of a circumstellar
envelope, its observable properties differ from those typical envelopes in Be
stars. b) Comparison with spectral standards and models indicates a possible
over-abundance in He. This would open the possibility to link the behavior of
BD+53$^\circ$2790 to the He-rich class of stars. c) UV spectra shows an
abnormally slow and dense wind for an O9.5V star. d) Spectral classification in
the IR wavelength region suggest a more likely supergiant nature of the source,
in contradiction with the optical classification. e) The presence of an intense
magnetic field is under investigation. BD+53$^\circ$2790 stands as a perfect
laboratory for testing stellar structure, as well as wind and evolutionary
theories. The observable properties of this source in a wide range of spectral
bands are discussed, and some interpretations outlined.
