Subdwarf B star

The hot subdwarfs are proposed to be the cause of the UV upturn in the light output of elliptical galaxies.

In the Palomar-Green survey, they were discovered to be the commonest kind of faint blue star with a magnitude over 18.

During the 1960s, spectroscopy discovered that many of the sdB stars are deficient in hydrogen, with abundances below that predicted by the Big Bang theory.

In the early 1970s Greenstein and Sargent measured temperatures and gravity strengths and were able to plot their correct position on the Hertzsprung–Russell diagram.

In plots of temperature against surface gravity, the short-period pulsators cluster together in the so-called empirical instability strip, approximately defined by T=28,000–35,000 K (27,700–34,700 °C; 49,900–62,500 °F) and log g=5.2–6.0.

V391 Pegasi was the first sdB star believed to have an exoplanet in orbit around it,[6][7] although subsequent research has significantly weakened the evidentiary case for the planet's existence.

These would have been engulfed by the red giant progenitor, and the rocky/metallic cores would be the only parts of the planets to survive without being evaporated.

2MASS J19383260+4603591 is the close binary system of a subdwarf B and a red dwarf star, which is orbited by the circumbinary planets Kepler-451b, Kepler-451c and Kepler-451d.