During the last years we undertook a programme to study Delta Scuti and/or RRc variable stars in order to obtain information about the modal content and/or Fourier parameters. Observations were carried out mainly at Merate Observatory and at ESO, but observing runs have also been carried out at Catania (UBV photometry) and Loiano (CCD photometry) Observatories.
This project led us to the discovery of unusual light curves (i.e. a descending branch steeper than the ascending one) of V1719 Cyg and V798 Cyg (Poretti and Antonello 1988, Astr. Ap. 199, 191). The former star certainly is a double-mode pulsator with a period ratio of 0.800, while the multiperiodicity of the other star is not yet definitely established. A third star showing this peculiarity, V974 Oph, is under investigation. It is probably a multimode pulsator and the multiperiodic behavior produces strong variations in the shape of the light curve. The reduction of these measurements carried out at ESO during April, 1989 allow us to obtain a more accurate picture of its modal content.
An adequate light curve solution was obtained tor the two monoperiodic Delta Scuti stars KU Cen (0.080 days) and V1162 Ori = BD -7°1108 (0.078 days). Together with HD 79989 (0.096 days) and BD +43°1984 (0.098 days) they were added to the high-amplitude Delta Scuti and SX Phe stars (Antonello et al. 1986, Astr. Ap. 169, 122) in order to study the structural properties of pulsating stars with periods less than 0.3 days (Poretti et al., Astr. Ap. in press).
Multiperiodic Delta Scuti stars with amplitudes between 0.05 and 0.10 mag were observed at ESO during November, 1987 and April, 1989. The power spectra ot o1 Eri, DL Eri and BK Cet are very rich and nonradial oscillations are probably present (Poretti, Astr. Ap. in press).
During Winter 1988/9, SAO 4710 was observed at Merate Observatory. The analysis of the 1020 measurements has led to the determination of a second period. The period ratio of (0.05839 days / 0.09874 days = 0.5913) rules out the existence of two radial modes. The corresponding Q values suggest that the first period (with full-amplitude of 0.032 mag) is due to the third radial overtone, while the second period (with full amplitude of 0.006 mag) is identified as a P1 nonradial mode of order l = 1 (Mantegazza and Poretti, Astr. Ap., in press).
The intensive observations of Delta Scuti stars will be continued with the same procedure new projects concerning faint, large-amplitude pulsators and medium-amplitude, multiperiodic variable stars have been started or proposed at ESO, while long-term programmes are being continued at Merate Observatory.