HD 166181 = V815 Herculis, a Single-lined Spectroscopic Multiple System

We have obtained extensive spectroscopic and velocity spectrometer observations of HD 166181, a previously known single-lined spectroscopic binary. Our improved orbit for the G6 V primary has a period of 1.8098343 days and is circular. Although the lines of additional components have not been detected, radial velocity measurements confirm that the system has additional velocity variations with a period of 2092 days, or 5.73 yr. This long-period orbit has an eccentricity of 0.76. An analysis of the Hipparcos observations produces a well-determined astrometric orbit for the long-period system that has an inclination of 78°. Mass estimates of the components in this zero-age main-sequence multiple system indicate that the unseen secondary in the 5.73 yr orbit may also be a binary. Thus, HD 166181 is at least a triple system and possibly quadruple.