The δ Scuti star FG Vir III.: The 1995 multisite campaign and the detection of 24 pulsation frequencies

We carried out the largest photometric multisite campaign for a delta Scuti star to date and acquired 435 hours of Stromgren v and y time-series photometry at 6 observatories during a time span of 40 days. The new 1995 data set allows us to extract 19 frequencies of the light variations. By including the data published by Breger et al. (1995) we can increase the number of significant frequencies of FG Vir to 24. The pulsation of FG Vir occurs in a frequency range from 9.2 to 34.1 c/d (106 to 395 mu Hz) with amplitudes as small as 0.4 mmag detected in y. Two peaks may be identified as combinations of other pulsation modes. There is strong evidence for the presence of further pulsation modes in the same frequency range. Their detection would require a data set with an even larger time base. An examination of all the available time-series photometric data to determine amplitude variability was also undertaken. Statistical tests show that only a single pulsation mode (f(3) = 23.40 c/d) is definitely variable and has changed in amplitude from 1.4 mmag in 1992, 2.3 mmag in 1993 to 4.1 mmag in 1995. Arguments are given that photometrically at least three different degrees, (I), have been detected. An upcoming paper will discuss possible mode identifications and pulsational model calculations.