Where are all the Sirius-like binary systems?

Approximately 70 per cent of the nearby white dwarfs appear to be single stars, with the being members of binary or multiple star systems. The most numerous and most identifiable systems are those in which the main-sequence companion is an M star, since if the systems are unresolved the white dwarf either dominates or is at least competitive the luminosity of the companion at optical wavelengths. Harder to identify are systems the non-degenerate component has a spectral type earlier than M0 and the white dwarf the less luminous component. Taking Sirius as the prototype, these latter systems referred to here as ‘Sirius like’. There are currently 98 known Sirius-like systems. Studies the local white dwarf population within 20 pc indicate that approximately 8 per cent of all dwarfs are members of Sirius-like systems, yet beyond 20 pc the frequency of known -like systems declines to between 1 and 2 per cent, indicating that many more of these remain to be found. Estimates are provided for the local space density of Sirius-like and their relative frequency among both the local white dwarf population and the population of A to K main-sequence stars. The great majority of currently unidentified -like systems will likely turn out to be closely separated and unresolved binaries. Ways to observationally detect and study these systems are discussed.