The variability code provides information on known or suspected
variable stars. Table V1 
lists the possible values
(all amplitudes are in 
):
[]
Details of the treatment of variable stars within the Hipparcos programme have been given by Mennessier & Figueras (1989) and Mennessier et al. (1992).
A blank field (not a known or suspected variable) does not mean that the star is not variable. The only stars known to be not variable at the level of about 0.02 mag are those with a flag C as photometric standard stars, as described by the second digit of field 26. Some stars were suspected variable by some observers, while subsequent observations have suggested that they are very stable. For such stars, their variability code may not be blank even though they are classified as photoelectric standard stars.
Code `3': stars in this category have been assigned a unique magnitude for the purposes of the Hipparcos observations, despite their (possibly large) amplitude variation. The definition of this unique magnitude depends on the type of variability (it may correspond to the maximum or minimum, or to a mean or weighted magnitude) and is specified by the second digit of field 44.
Code `4': for these large-amplitude variable stars, the use
of an ephemeris was necessary for specifying the observing time to be
allocated to the Hipparcos observations. In this case, a weighted
magnitude is given in field 25. As specified by the second digit of
field 44, the weighted magnitude used for stars in this category
corresponds to:
[
magnitude at maximum luminosity 
magnitude at minimum luminosity].
Code `5': these stars, for which the amplitude of variation
is smaller than 0.2 mag, are processed in the same way as non-variable
stars.
