For nine variables Hipparcos made simultaneous (within 1d) or nearly
simultaneous (within 2d) observations with other photometric systems, viz.
the Strömgren uvby-, Walraven VBLUW-, or Johnson UBV photometric
systems. Magnitudes in visual light obtained with the first two systems were
transformed to the Johnson 
of the UBV system.
The Hp passband is very wide and almost covers the three Johnson passbands,
but has an effective wavelength very close to .
Consequently, some difference between the two magnitude scales can be expected
because of a possible dependence on the colours of the star. Small differences
can be also expected due to light variations within 
(
),
although the program stars have in most cases periods longer than 10d.
Such differences were discribed in ESA 1997, Vol. 1, Sect. 1.3
in a general comparison between Hp and magnitudes.
Table 1 (click here) lists the nine stars with their spectral type.
The third and fourth column list the difference 
= 
and
the number (n) of observations used, respectively. The standard deviation
(
) of is of the order of a few 001.
These differences are mostly in good agreement with the curve shown in
Fig. 1.3.4 of the above mentioned volume. The main difference is for HIP 67261, where a
value of +0.15 would have been expected, but this can be attributed to the use
of the wrong colour index in the Hipparcos data reductions.
A second method is to compare both magnitude scales with the aid of the mean
magnitudes, at least if sufficient data are known and if the star does
not show a very long-time scale variability. Also when the range
is large and the number of data is too low, no value is given.
Tables 2 (click here) and 3 (click here) (to be discussed in Sect. 5) lists these
values.