very early spectral type O7.5III. The remaining star is either a blue giant or supergiant of a late O or early B spectral type. Also an eclipsing binary, the HD 166734 system consists of two O-type blue supergiants in orbit around each other. Less extreme in terms of mass and temperature is HD 161701, a spectroscopic binary consisting of a B-type primary and an Ap secondary, although it is the only known spectroscopic binary to consist of a star with excess of mercury and manganese and an Ap star. South of the Eagle Nebula on the border with Sagittarius is the eclipsing binary W Serpentis, whose primary is a white giant that is interacting with the secondary. The system has been found to contain an accretion disk, and was one of the first discovered Serpentids, which are eclipsing binaries containing exceptionally strong far-ultraviolet spectral lines. It is suspected that such Serpentids are in an earlier evolutionary phase, and will evolve first into double periodic variables and then classical Algol variables. Also near the Eagle Nebula is the eclipsing Wolf–Rayet binary CV Serpentis, consisting of a Wolf–Rayet star and a hot O-type subgiant. The system is surrounded by a ring-shaped nebula, likely formed during the Wolf–Rayet phase of the primary. The eclipses of the system vary erratically, and although there are two theories as to why, neither of them is completely consistent with current understanding of stars. Serpens Cauda contains a few X-ray binaries. One of these, GX 17+2, is a low-mass X-ray binary consisting of a neutron star and, as in all low-mass X-ray binaries, a low-mass star. The system has been classified as a Sco-like Z source, meaning that its accretion is near the Eddington limit. The system has also been found to approximately every 3 days brighten by around 3.5 K-band magnitudes, possibly due to the presence of a synchrotron jet. Another low-mass X-ray binary, Serpens X-1, undergoes occasional X-ray bursts. One in particular lasted nearly four hours, possibly explained by the burning of carbon in "a heavy element ocean". Φ 332 (Finsen 332) is a tiny and difficult double-double star at 18:45 / +5°30', named Tweedledee and Tweedledum by South African astronomer William Stephen Finsen, who was struck by the nearly identical position angles and separations at the time of his 1953 discovery. Gliese 710 is a star that is expected to pass very close to the