A MULTISITE UBV PHOTOMETRIC CAMPAIGN ON 53-PERSEI IN 1991 JANUARY

New UBV observations of 53 Persei obtained during an international pho tometric campaign in 1991 January are presented. Techniques of observa tion and data reduction employed at each of the four participating obs ervatories are described in detail, followed by a discussion of the ob servational uncertainties and systematic errors. The formal standard d eviations determined from check-star measurements are 0.011, 0.012, an d 0.020 mag for V, B, and U, respectively. Multiple frequency analysis shows that the light variations of 53 Persei during the campaign can be fitted very well by two sinusoids of frequencies 0.462 and 0.603 cy cles per day (c/d) which are practically identical to those found by o thers from 1977-1983 photometric and spectroscopic data. The detection of the same frequencies in the data obtained more than 10 yr apart ha s convincingly confirmed the stable multiperiodicity in 53 Persei and extended the stability duration from 5.5 yr reported in an earlier inv estigation to about 13 yr. This pivotal result strongly supports the n onradial pulsation as the physical cause responsible for light and lin e-profile variations in this prototype star. Definite changes have bee n detected in amplitudes and amplitude ratios of the light variations. Since 1981, the amplitude ratio of the two modes mentioned above has increased from approximately 0.9 to > 2.5. The single mode of 0.462 c/ d has become the dominant component in the two-mode pulsation of 53 Pe rsei. The deviations of data from the two-frequency fits may imply the existence of additional frequencies. But we cannot prove this inferen ce with the present data.