HW Virginis

HW Virginis is an eclipsing binary star system located in the constellation Virgo. It is classified as a post-common-envelope binary (PCEB), consisting of a hot subdwarf B star (sdB star) and a red dwarf companion orbiting each other in a very close, short-period orbit.

The sdB star in HW Virginis is a compact, helium-burning star with a high surface temperature. The red dwarf companion is a low-mass, cool star, likely tidally locked to the sdB star. The system's orbital period is extremely short, approximately 0.116 days (or about 2 hours and 47 minutes). This close proximity results in strong tidal forces and gravitational interactions between the two stars.

HW Virginis exhibits deep eclipses as the red dwarf passes in front of the sdB star, as viewed from Earth. These eclipses provide valuable information about the stars' sizes, masses, and orbital parameters.

The system is known to host at least two circumbinary planets, designated HW Virginis b and HW Virginis c. These planets were detected through the analysis of eclipse timing variations (ETVs), which are subtle changes in the timing of the eclipses caused by the gravitational influence of the orbiting planets. The presence of circumbinary planets in such a close binary system is significant for understanding planet formation and evolution in extreme environments. The detection method is based on O-C (observed minus calculated) diagrams, showcasing the difference between the predicted eclipse times and the actual eclipse times. This discrepancy points to a light-travel time effect due to the movement of the binary system around the center of mass of the triple (or higher-order) system.

The study of HW Virginis and similar PCEBs contributes to our understanding of stellar evolution, binary star interactions, circumbinary planets, and the long-term fate of planetary systems in dynamically active environments.