Fields 39-44: Variability

[]Variable stars, especially those with large amplitude, raise specific problems for the Hipparcos observations. The optimum `target observing time' is determined from the Hipparcos magnitude, and this observing time varies strongly as a function of magnitude. The allocation of excessive observing time to a star is inefficient, while allocating too short an observing time would result in less accurate astrometric parameters at the end of the mission (Mennessier & Baglin 1988). Ephemerides are therefore used for large-amplitude variable stars, while a unique magnitude is chosen for the others (Mennessier 1985). The limiting amplitude for requiring the use of ephemerides was taken as mag. For stars with a smaller amplitude, a unique magnitude was chosen according to the type of variability. The choice is indicated as a code in the second digit of field 44.

• Field 39: Variable star name
• Field 40: Type of variability
• Field 41: Period of variation
• Field 42: magnitude at maximum luminosity
• Field 43: magnitude at minimum luminosity
• Field 44, 1st digit: Coded error of the magnitudes at maximum/minimum luminosity
• Field 44, 2nd digit: Code specifying the magnitudes and colour given in fields 25, 27, 29