X1908 + 075: An X-ray binary with a 4.4 day period

X1908 + 075 is an optically unidentified and highly absorbed X-ray source that appeared in early surveys such as Uhuru, OSO 7, Ariel 5, HEAO-1, and the EXOSAT Galactic Plane Survey. These surveys measured a source intensity in the range 2-12 mcrab at 2-10 keV, and the position was localized to ∼ 0°.5. We use the Rossi X-Ray Timing Explorer (RXTE) All-Sky Monitor (ASM) to confirm our expectation that a particular Einstein/IPC detection (1E 1908.4 + 0730) provides the correct position for X1908 + 075. The analysis of the coded mask shadows from the ASM for the position of IE 1908.4 + 0730 yields a persistent intensity ∼ 8 mcrab (1.5-12 keV) over a 3 yr interval beginning in 1996 February. Furthermore, we detect a period of 4.400 ± 0.001 days with a false-alarm probability less than 10^-7. The folded light curve is roughly sinusoidal, with an amplitude that is 26% of the mean flux. The X-ray period may be attributed to the scattering and absorption of X-rays through a stellar wind combined with the orbital motion in a binary system. We suggest that X1908 + 075 is an X-ray binary with a high-mass companion star.