The chromospherically active single star HD 181943 was previously proposed as either a pre-main-sequence star seen pole-on or a post-main-sequence subgiant that evolved from an Ap star similar to 53 Cam. It also was found to be a variable star with a 385 day period. We have used new spectroscopic and photometric observations to reevaluate the properties and evolutionary state of this unusual system. We conclude that HD 181943 is a single K1 V star that has recently arrived on the ZAMS. Its emission line fluxes are nearly identical to those of HD 82558= LQ Hya, while its log lithium abundance of 1.75 is somewhat smaller than that of similar Pleiades stars. Its low v sin i of 5 km s-1 suggests that its rotational inclination axis is likely ≤12°. We estimate a distance of 46±6 pc or a parallax of 0.022". Space velocities indicate that it belongs to no known moving group. Our photometric observations, covering nearly 2300 days, confirm that HD 181943 is variable. However, they do not support the 385 day period but instead suggest a period of about 2000 days. When photometry from earlier epochs is included no single period fits all the data. We interpret this as evidence for changes in high latitude or polar spots. Portions of our photometric data analyzed for rotational modulation reveal no periodic short-term variations. Continued observation of the comparison star, HD 181219=V4153 Sgr, indicates that it is a constant star and so its variable-star classification should be revised. Variability previously ascribed to this star was the result of moonlight contamination.
Fekel, F.C., Henry, G.W., Hall, D.S. "Chromospherically Active Stars. XIV. A Rediscussion of the Properties and Evolutionary State of HD 181943" Astronomical Journal v.109, p.2821 (1995)