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The main goal of this project is to search for p-mode oscillations in aselected sample of DA white dwarfs near the blue edge of the DAV (g-mode)instability strip, where the p-modes should be excited following theoreticalmodels. A set of high quality time-series data on nine targets has beenobtained in 3 photometric bands (Sloan u', g', r') using ULTRACAM at the VLTwith a typical time resolution of a few tens of ms. Such high resolution isrequired because theory predicts very short periods, of the order of a second,for the p-modes in white dwarfs. The data have been analyzed using Fouriertransform and correlation analysis methods. Results: P-modes have not beendetected in any of our targets. The upper limits obtained for the pulsationamplitude, typically less than 0.1%, are the smallest limits reported in theliterature. The Nyquist frequencies are large enough to fully cover thefrequency range of interest for the p-modes. For the brightest target of oursample, G 185-32, a p-mode oscillation with a relative amplitude of 5x10**(-4)would have been easily detected, as shown by a simple simulation. For G 185-32we note an excess of power below ~2 Hz in all the three nights of observation,which might be due in principle to tens of low-amplitude close modes. However,neither correlation analysis nor Fourier transform of the amplitude spectrumshow significant results. We also checked the possibility that the p-modes havea very short lifetime, shorter than the observing runs, by dividing each run inseveral subsets and analyzing these subsets independently. The amplitudespectra show only a few peaks with S/N ratio higher than 4 sigma but the samepeaks are not detected in different subsets, as we would expect, and we do notsee any indication of frequency spacing. As a secondary result of this project,... (see the paper)

Search for p-mode oscillations in DA white dwarfs with VLT-ULTRACAM. I. Upper limits to the p-modes