AR Aurigae

Lore & Background

The two inner components of AR Aurigae form an eclipsing binary system, causing its brightness to vary from magnitude +6.15 to +6.82 with a period of 4.13 days. Both stars are blue-white B-type main-sequence stars that do not fill their Roche lobes. The primary component is a mercury-manganese (HgMn) star, characterized by over-abundances of mercury, manganese, and often xenon and other elements. Because AR Aurigae is an eclipsing binary, accurate characterization of its parameters has been made possible.

The eclipses were first detected in 1931 by H. N. Pendersen and J. C. Steensgard. When an eclipse is not occurring, the star is faintly visible to the naked eye under very good observing conditions. A third component has been detected by analyzing the difference between observed and predicted eclipse times, caused by the light-time effect of its orbital motion around the inner pair. This star has a mass that is not specified in the available data and orbits at a separation of 13 AU every 23.7 years.

Reader's Guide

AR Aurigae holds a unique place in stellar astrophysics as the only known eclipsing binary system that includes a mercury-manganese star. This rare combination allows astronomers to derive precise physical parameters for an HgMn star—such as mass, radius, and temperature—that are difficult to obtain for single stars. The eclipses provide a direct geometric method to measure the stars' properties, which in turn helps refine models of chemical peculiarity in B-type stars. The system's triple nature, revealed through the light-time effect on eclipse timings, adds further complexity and offers a laboratory for studying orbital dynamics and stellar evolution in multiple systems. Its variability, with a period of 4.13 days and a magnitude range of 0.67, makes it a well-characterized target for both professional and advanced amateur observers. The discovery of its eclipses in 1931 by Pendersen and Steensgard marked the beginning of its scientific importance, which continues to this day as a benchmark for understanding chemically peculiar stars in binary systems.

Did You Know?

• AR Aurigae is the only known eclipsing binary that contains a mercury-manganese star.
• Its brightness varies from magnitude +6.15 to +6.82 every 4.13 days.
• The third component was detected via the light-time effect on eclipse timings.