RX Andromedae

Lore & Background

English amateur astronomer Arthur Stanley Williams discovered RX Andromedae in 1904, observing it visually with a 6.5 inch reflector from late 1904 until early 1905. He initially believed it to be a Cepheid variable, but after examining images on photographic plates dating back to late 1899, he concluded it was a U Geminorum variable. He published its variable star designation in 1906.

The system is a cataclysmic variable consisting of a white dwarf with a mass of 0.8 M☉ and an M2 main sequence star rotating around their center of mass. The main sequence star overfills its Roche lobe, so the white dwarf strips away matter and accretes it through an accretion disk.

Like Z Camelopardalis variables, RX Andromedae shows periods of roughly constant luminosity and others where its brightness oscillates between magnitude 10.2 at maximum and 15.1 at minimum. However, between 1996 and 1997 it was stuck at its minimum brightness like VY Sculptoris variables, before returning to usual behavior. The white dwarf and its accretion disk appear entirely responsible for this variability, driven by changes in the accretion rate.

Reader's Guide

RX Andromedae holds significance as a rare transitional object between two major classes of cataclysmic variables: Z Camelopardalis dwarf novae and VY Sculptoris stars. Its behavior—mostly oscillating between magnitude 10.2 and 15.1 with a 13-day period, but occasionally entering prolonged low-luminosity states—provides insight into the mechanisms driving accretion variability in close binary systems. The star has been extensively studied in optical and ultraviolet wavelengths, and it is one of the few dwarf nova systems detected at radio wavelengths. Its discovery by Arthur Stanley Williams in 1904, initially mistaken for a Cepheid, highlights the evolving understanding of variable star classification. The system's transitional nature, confirmed by its 1996–1997 minimum-brightness episode, underscores the complexity of accretion processes in cataclysmic variables and the role of the white dwarf's accretion rate in governing outburst behavior.

Did You Know?

• RX Andromedae was discovered in 1904 by English amateur astronomer Arthur Stanley Williams.
• Between 1996 and 1997 it remained at minimum brightness, behaving like a VY Sculptoris star.
• The white dwarf has a mass of 0.8 M☉ and accretes matter from an M2 main sequence companion.