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 obtained of \sim 16M _ { \odot } together with the detection of an extended near-total eclipse confirm that the primary star is a supergiant as predicted .
The orbital eccentricity is found to be very low with a best fit value of 0.04 \pm 0.02 .
The orbital period is also refined to be 6.0724 \pm 0.0009 days using an improved and extended light curve obtained with RXTE ’ s All Sky Monitor .
Observations with the ASCA satellite provide an improved source location of R.A. = 18 ^ { h } 55 ^ { m } 31.3 ^ { s } , decl .
= -02 ^ { \circ } 36′ 24.0″ ( 2000 ) with an estimated systematic uncertainty of less than 12″ .
A serendipitous new source , AX J1855.4-0232 , was also discovered during the ASCA observations .