A subdwarf (sd) is a star with less luminosity than is typical for its spectrum. One criteria is with an absolute magnitude of at least 1.5 higher. The luminosity class VI indicates a subdwarf. An example is Kapteyn's Star. For early stars, this is presumed to be due to mass loss, perhaps in some cases from binary-star interaction. For later stars, it occurs when metallicity is low, suggesting that Population II stars were smaller given their mass. Lower opacity decreases outward radiation pressure and they are less puffed up, with a hotter photosphere for their given radius. Some late subdwarfs have other characteristics associated with old stars.
A number of a particular kind of B-type subdwarfs are about 0.5 Solar mass, with radius on the order of 0.2 solar radii, which are presumed to be red giants that lost their outer layers by some mechanism, possibly stellar interactions.