THE ORBITAL PERIOD OF SWIFT J1626.6-5156

We present the discovery of the orbital period of Swift J1626.6-5156. Since its discovery in 2005, the source has been monitored with Rossi X-Ray Timing Explorer, especially during the early stage of the outburst and into the X-ray modulating episode. Using a data span of similar to 700 days, we obtain the orbital period of the system as 132.9 days. We find that the orbit is close to a circular shape with an eccentricity 0.08, that is one of the smallest among Be/X-ray binary systems. Moreover, we find that the timescale of the X-ray modulations varied, which led to earlier suggestions of orbital periods at about a third and half of the orbital period of Swift J1626.6-5156.