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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