The photometric system utilized has the advantage, that the uvby
photometry is acquired simultaneously
and the N and W filters that define almost simultaneously.
All the observations were carried out at the Observatorio Astronómico
Nacional at San Pedro Mártir, Mexico. The data was acquired in
different observing seasons: July-August, 1986; August-September,
1986; September-October, 1992 and March, 1994. All were carried out at
the 1.5 m telescope with the same spectrophotometer. A description of
this instrument can be found in Nissen (1984). The reduction procedure
was done through the numerical packages NABAPHOT (Arellano-Ferro &
Parrao 1989) and DAMADAP (Parrao et al. 1998) which reduce the data
into a standard system. The system that was chosen was that defined by
the standard values of Olsen (1983) and the transformation equations
are those defined by Crawford & Barnes (1970) and by Crawford &
Mander (1966). An estimate of accuracy was done by comparing
the
data obtained with
those of Olsen (1983) and Grönbech & Olsen (1977). The
uncertainties were evaluated in the following manner: The average
differences, present data of the standard stars minus Olsen's data,
were evaluated and
provide an uncertainty for the transformation of the season, see Table
1. These values, of course, are
functions of the star magnitude and in all cases enough star counts
were secured in order to attain a signal to noise ratio large enough
to determine an accuracy better than 0.01 mag, small compared to the
large amplitude variation of these stars. The final photometric values have been submitted to the IAU archive but the mean Strömgren indices which
cover a whole cycle and the range of the variation due to pulsation for each star are listed in Table 2.
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