We present observations of the 9.8 s X-ray pulsar XTE J1859+083 made with the ASM and PCA on board RXTE , and the WFC on board BeppoSAX .
The ASM data cover a 12 year time interval and show that an extended outburst occurred between approximately MJD 50,250 and 50,460 ( 1996 June 16 to 1997 January 12 ) .
The ASM data excluding this outburst interval suggest a possible modulation with a period of 60.65 \pm 0.08 days .
Eighteen sets of PCA observations were obtained over an approximately one month interval in 1999 .
The flux variability measured with the PCA appears consistent with the possible period found with the ASM .
The PCA measurements of the pulse period showed it to decrease non-monotonically and then to increase significantly .
Doppler shifts due to orbital motion rather than accretion torques appear to be better able to explain the pulse period changes .
Observations with the WFC during the extended outburst give a position which is consistent with a previously determined PCA error box , but which has a significantly smaller error .
The transient nature of XTE J1859+083 and the length of its pulse period are consistent with it being a Be/neutron star binary .
The possible 60.65 day orbital period would be of the expected length for a Be star system with a 9.8 s pulse period .