1 Introduction

Asteroseismology of $\delta$ Scuti stars has reached a stage where the choice between different models of stellar structure and evolution require a large number of known pulsation frequencies. Recent examples of comparisons between observed and modelled nonradial multiple frequencies can be found in Pamyatnykh et al. (1998), Breger et al. (1998), Viskum et al. (1998) and Guzik, Templeton & Bradley (1998). The detection of the frequencies presents a severe challenge to the observers: the most commonly applied method utilizes extensive multisite campaigns, where millimag photometric accuracy is obtained. To obtain the required instrumental stability, the Delta Scuti Network adopts the three-star technique (Breger 1993), where measurements of the variable star are alternated with those of two comparison stars. The stars are usually changed manually at the telescope approximately every 90 s.

The conventional technique provides a heavy burden on the observer: the large amount of required data means work of weeks or months at an observatory, which can be quite expensive. Our method of observation with the three-star technique is perfectly suited for an Automatic Photoelectric Telescope (APT), if sufficient accuracy can be obtained without observer intervention. The present paper reports a successful application of APT technology to $\delta$  Scuti stars.

The variability of the $\delta$ Scuti star 4 CVn (HR 4715 = HD 107904 = AI CVn, F3III-IV) was discovered by Jones & Haslam (1966). The discovery was followed by a number of usually relatively short observational studies with contradictory frequency solutions caused, in part, by the lack of data. During the years 1983 and 1984 a multisite campaign by the Delta Scuti Network (Breger et al. 1990, Paper I) was carried out at four collaborating observatories. This led to the determination of five pulsation frequencies which are free of 1 cd^-1 alias problems. The analysis of the campaign data together with the previously (mostly unpublished) photometry revealed that 4 CVn pulsates with at least seven frequencies with values of 8.59, 7.37, 6.98, 6.19, 5.85, 5.53, and 5.05 cd^-1. These seven frequencies are independently found and confirmed in different subsets of the 114 nights of photometric data covering the years 1966 to 1984 (Breger 1990a, Paper II).

Extensive series of unpublished photometry (see Fitch 1980) also showed two unusually small frequencies. 288 hours of new high-quality observations during a 58-day time span were obtained by Breger et al. (1997) to solve this question. The two low frequencies of 1.32 and 1.40 cd^-1 were found to originate in the comparison star used in many studies, HD 108100, which was hereby discovered to be a $\gamma$ Doradus g-mode pulsator.