The APT measurements have increased the number of known pulsation frequencies of 4 CVn tremendously from 7 to 19, of which 6 are linear combination frequencies. All of the previously detected frequencies were confirmed with high amplitudes of 5 mmag or more. The newly detected frequencies have smaller amplitudes, except for f4, which has appeared with a very high amplitude of 10 mmag in y and 16 mmag in v. Although f6 and f4 are separated by only 0.07 cd-1, the resolution of the older data in the literature is sufficient to separate the two modes unambiguously. Inspection of the older data indicates that this pulsation mode was either not present earlier or had an amplitude of 3 mmag or less.
This growth of a "new" pulsation mode is not unusual for 4 CVn. The star has a history of slow but steady amplitude changes with a time scale of years (see Breger 1990a). Nevertheless, more spectacular changes also occur: from 1974 to 1976/7, the high-amplitude mode f2 essentially disappeared in order to slowly grow again in amplitude during the next decade.
It was shown that the presently available APT is able to measure at the millimag level. For differences, C1-C2, in the y and v filters, residuals of 2.65 and 3.00 mmag were found, respectively. 32 nights of data of the Scuti variable 4 CVn made it possible to extract 19 simultaneously excited frequencies with amplitudes as small as 1 mmag. The 19-frequency fit of 4 CVn leaves residuals of 2.98 and 4.03 mmag. These residuals are slightly higher than those of the comparison stars and indicate the presence of additional pulsation modes with small amplitudes. The presence of additional, undetected modes is not in contradiction to the excellent fit of the observed and computed light curves shown in Fig. 3. The visible light curve is comprised mainly of the modes with large amplitudes, viz. f1 to f8. Large amounts of high-precision data allow the extraction of additional modes whose detection is statistically significant while not changing the quality of the computed light curves to a noticeable degree.
Part of the investigation has been supported by the Austrian Fonds zur Förderung der wissenschaftlichen Forschung, project numbers S7304 and S7301.
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