  With recent and archival Rossi X-Ray Timing Explorer (RXTE) X-ray
measurements of the heavily obscured X-ray pulsar IGR J16393-4643, we carried
out a pulse timing analysis to determine the orbital parameters. Assuming a
circular orbit, we phase-connected data spanning over 1.5 years. The most
likely orbital solution has a projected semi-major axis of 43 +- 2 lt-s and an
orbital period of 3.6875 +- 0.0006 days. This implies a mass function of 6.5 +-
1.1 M_sun and confirms that this INTEGRAL source is a High Mass X-ray Binary
(HMXB) system. By including eccentricity in the orbital model, we find e < 0.25
at the 2 sigma level. The 3.7 day orbital period and the previously known ~910
s pulse period place the system in the region of the Corbet diagram populated
by supergiant wind accretors, and the low eccentricity is also consistent with
this type of system. Finally, it should be noted that although the 3.7 day
solution is the most likely one, we cannot completely rule out two other
solutions with orbital periods of 50.2 and 8.1 days.
