V391 Pegasi

Subdwarf B stars such as V391 Pegasi are thought to be the result of the ejection of the hydrogen envelope of a red giant star at or just before the onset of helium fusion. The ejection left only a tiny amount of hydrogen on the surface—less than 1/1000 of the total stellar mass. The future for the star is to eventually cool down to make a low-mass white dwarf. Most stars retain more of their hydrogen after the first red giant phase, and eventually become asymptotic giant branch stars. The reason that some stars, like V391 Pegasi, lose so much mass is not well known. At the tip of the red-giant branch, the red giant precursors of the subdwarf stars reach their maximum radius, on the order of 0.7 AU. After this point, the hydrogen envelope is lost and helium fusion begins—this is known as the helium flash. == Hypothesized planetary system ==

In 2007, research using the variable star timing method indicated the presence of a gas giant planet orbiting V391 Pegasi. However, subsequent research published in 2018, taking the large amount of new photometric time-series data amassed since the publication of the original data into account, found evidence both for and against the exoplanet's existence. Although the planet's existence was not disproved, the case for its existence was now certainly weaker, and the authors stated that it "requires confirmation with an independent method." == References ==