Z Tauri is a Mira variable with a period of 466 days, and an amplitude of about four magnitudes... or so we thought! In reality, Z Tau is a star with a nearby constant star within 10 arcseconds that most observers can't resolve. When Z Tau goes into decline and falls below magnitude 14, most observers see (and report) the nearby star. As a result, the light curve of Z Tau winds up with a flat bottom. In reality, Z Tau declines to about magnitude 18, making Z Tau one of the larger-amplitude Miras known, and nearly rivalling that of the majestic khi Cygni. The problem of flat-bottomed light curves was first noted by AAVSO observer Tom Cragg in a talk entitled "Sanctum Observers and the Minima of the Mira Stars" given at the Spring 1963 AAVSO meeting in Long Beach, California. Observers with larger aperture telescopes and exceptional seeing conditions may be able to separate Z Tau from the nearby star, but even then, it's a challenge! Walter Cooney and collaborators followed Z Tau through minimum with the Sonoita 35-cm telescope along with telescopes at the Naval Observatory in Flagstaff, and you can follow it too if you can resolve the close pair and go below 17th magnitude.
Prepared By: Matthew Templeton