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R. Corbet (NASA GSFC/USRA)
A pulse timing orbit has been obtained for the X-ray binary XTE J1855-026 using observations made with the Proportional Counter Array on board the Rossi X-ray Timing Explorer. The mass function of approximately 16 Mo confirms that the primary star is a supergiant as previously predicted based on the source's location in the orbital period/pulse period diagram. The light curve shows the presence of a near-total eclipse with a duration of greater than 0.4 days. As well as confirming the supergiant classification, this eclipse gives additional constraints on the orbital parameters. No significant orbital eccentricity is found (e = 0.03 ± 0.02). The orbital period is also refined using additional observations with RXTE's All Sky Monitor.