4U 2206+54: An unusual high-mass X-ray binary with a 9.6 day orbital period but no strong pulsations

Rossi X-Ray Timing Explorer (RXTE) All-Sky Monitor observations of the X-ra y source 4U 2206+54, previously proposed to be a Be star system, show the X -ray flux to be modulated with a period of approximately 9.6 days. If the m odulation is due to orbital variability, then this would be one of the shor test orbital periods known for a Be star X-ray source. However, the X-ray l uminosity is relatively modest, whereas a high luminosity would be predicte d if the system contains a neutron star accreting from the denser inner reg ions of a Be star envelope. Although a 392 s pulse period was previously re ported from EXOSAT observations, a reexamination of the EXOSAT light curves does not show this or any other periodicity. An analysis of archival RXTE Proportional Counter Array observations also fails to show any X-ray pulsat ions. We consider possible models that may explain the properties of this s ource, including a neutron star with accretion halted at the magnetosphere and an accreting white dwarf.