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

A multiwavelength approach to BD+53$^\circ$2790: the O9.5V counterpart to the X-ray binary system 4U~2206+54