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We present observations of the 9.8 s X-ray pulsar XTE J1859+083 made with theASM and PCA on board RXTE, and the WFC on board BeppoSAX. The ASM data cover a12 year time interval and show that an extended outburst occurred betweenapproximately MJD 50,250 and 50,460 (1996 June 16 to 1997 January 12). The ASMdata excluding this outburst interval suggest a possible modulation with aperiod of 60.65 +/- 0.08 days. Eighteen sets of PCA observations were obtainedover an approximately one month interval in 1999. The flux variability measuredwith the PCA appears consistent with the possible period found with the ASM.The PCA measurements of the pulse period showed it to decreasenon-monotonically and then to increase significantly. Doppler shifts due toorbital motion rather than accretion torques appear to be better able toexplain the pulse period changes. Observations with the WFC during the extendedoutburst give a position which is consistent with a previously determined PCAerror box, but which has a significantly smaller error. The transient nature ofXTE J1859+083 and the length of its pulse period are consistent with it being aBe/neutron star binary. The possible 60.65 day orbital period would be of theexpected length for a Be star system with a 9.8 s pulse period.

RXTE and BeppoSAX Observations of the Transient X-ray Pulsar XTE J1859+083