  Massive X-ray binaries are usually classified depending on the properties of
the donor star in classical, supergiant and Be X-ray binaries. The massive
X-ray binary 4U 2206+54 does not fit in any of these groups, and deserves a
detailed study to understand how the transfer of matter and the accretion on to
the compact object take place. To this end we study an IUE spectrum of the
donor and obtain a wind terminal velocity (v_inf) of ~350 km/s, which is
abnormally slow for its spectral type. We also analyse here more than 9 years
of available RXTE/ASM data. We study the long-term X-ray variability of the
source and find it to be similar to that observed in the wind-fed supergiant
system Vela X-1, reinforcing the idea that 4U 2206+54 is also a wind-fed
system. We find a quasi-period decreasing from ~270 to ~130 d, noticed in
previous works but never studied in detail. We discuss possible scenarios and
conclude that long-term quasi-periodic variations in the mass-loss rate of the
primary are probably driving such variability in the measured X-ray flux. We
obtain an improved orbital period of 9.5591 d with maximum X-ray flux at MJD
51856.6. Our study of the orbital X-ray variability in the context of wind
accretion suggests a moderate eccentricity around 0.15. Moreover, the low value
of v_inf solves the long-standing problem of the relatively high X-ray
luminosity for the unevolved nature of the donor, BD +53 2790, which is
probably an O9.5 V star. We note that changes in v_inf and/or the mass-loss
rate of the primary alone cannot explain the diferent patterns displayed by the
orbital X-ray variability. We finally emphasize that 4U 2206+54, together with
LS 5039, could be part of a new population of wind-fed HMXBs with main sequence
donors, the natural progenitors of supergiant X-ray binaries. (Abridged)
