Mira B, also known as VZ Ceti, is the companion star to the variable star Mira, probably a white dwarf. The two are separated by around . Suspected as early as 1918, it was visually confirmed in 1923 by Robert Grant Aitken, and has been observed more or less continually since then, most recently by the Chandra X-Ray Observatory.
Long known to be erratically variable itself, its fluctuations seem to be related to its accretion of matter from Mira's stellar wind, which makes it a symbiotic star.
The exact orbit around Mira is poorly known due to its long period, though a commonly cited estimate of 497.9 years was published by Prieur et al in 2002. Assuming the distance in the Hipparcos catalog and orbit are correct, Mira A and B are separated by an average of 100 AU.
In January 2007, astronomers at the Keck Observatory announced the discovery of a protoplanetary disk around Mira B. Discovered via infrared data, the disk is apparently derived from captured material from Mira itself; Mira B accretes as much as one percent of the matter lost by its primary. Though planetary formation is perhaps unlikely as long as the disk is in active accretion, it may proceed apace once Mira A completes its red giant phase and becomes a white dwarf.
Mira B was long suspected to itself be a white dwarf. Several factors, such as its low x-ray luminosity, suggested that Mira B could instead be a normal main-sequence star of spectral type K and with a mass of roughly . However, a 2010 analysis of rapid optical brightness variations indicated that Mira B is, in fact, a white dwarf. Although most white dwarfs have masses close to , Mira B's accretion rate suggest a mass of either in the case of accretion through overflow of the red giant's Roche lobe by its stellar wind, or in the case of BondiâÂÂHoyleâÂÂLyttleton accretion. Mass-radius relations give radii of , respectively. The case of wind roche lobe overflow is more probable, so Mira B is likely an extremely low-mass white dwarf.