Orbital Separations of White Dwarf--Red Dwarf Binaries

We measured radial velocities of 21 white dwarf-red dwarf binaries to learn whether their components are close enough now to have shared a common envelope sometime in the past. Of the 14 for which we have adequate data, five are close binaries, while the other nine appear to be widely separated with no previous common envelope evolution. For the former five, some mass transfer likely occurred, so the current masses of the corresponding red dwarfs may vary from their initial masses, and this should be accounted for in derivations of the initial mass function for star formation at the very low-mass end of the main sequence.