A letter (or letters) in this field indicates that the star is part of a known double or multiple system. In the case of a double system with well-separated components, for example, `A' or `B' indicates that the entry corresponds to that component. In the case of a `joint' entry (see field 3) letters indicate which of the components of the system are considered (e.g. `AB', `AC', `APB', etc.).
[]Due to the nature of the Hipparcos detection system (the
diameter of the instantaneous field of view of the primary detection
system is about 38 arcsec), two stars or components within 10 arcsec
are observed by the satellite as a unique target. If the component
separation is larger than 10 arcsec, the number of discrete entries in
the main catalogue will correspond to the number of separate observing
targets defined within the observing programme. The number of such
entries will not necessarily correspond to the number of known
components of the double or multiple system, for example in those cases
where the fainter component(s) have not been retained for the satellite
observations.