3U 1700-37 = HD 153919: review.

X-ray, spectroscopic, and photometric data for the source are reviewed briefly. Some points of controversy and difficulty are discussed. The X-ray source 3U 1700-37 is moderately strong (approximately 100 Uhuru counts) and shows a long eclipse, with a period of 3.412 days. The X-rays are attenuated near 0.5 phase, and show a very wide gradual decrease on either side of the total eclipse. The low-energy cutoff is the strongest of all the X-ray binaries. These characteristics are all qualitatively compatible with the optical star's, which represent an extreme in several ways: (1) the primary is an extreme star with large mass outflow through a spherically symmetric stellar wind; (2) it is the hottest of all X-ray binary primaries (with possible exception of Cen X-3); and (3) the mass ratio is very high and the relative separation of the stars is low.