These stars rotate extremely rapidly (~100 km/s at the equator); hence they are ellipsoidal in shape. They are (apparently) single giant stars with spectral types G and K and show strong chromospheric emission lines. Examples are FK Com, V1794 Cygni and UZ Librae. A possible explanation for the rapid rotation of FK Comae stars is that they are the result of the merger of a (contact) binary.