A member of the cluster NGC 6604,[72] the two eclipsing stars are both blue giants, with one being of the 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.[73] Also an eclipsing[74] binary, the HD 166734 system consists of two O-type blue supergiants in orbit around each other.[75] 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.[76]