  Statistical analyses of several Delta Scuti stars (FG Vir, 44 Tau, BL Cam and
others) show that the photometrically observed frequencies cluster around the
frequencies of the radial modes over many radial orders. The observed
regularities can be partly explained by modes trapped in the stellar envelope.
This mode selection mechanism was already proposed by Dziembowski & Krolikowska
(1990) and was shown to be efficient for l=1 modes. New pulsation model
calculations confirm the observed regularities. We present the s-f diagram,
which compares the average separation of the radial frequencies (s) with the
frequency of the lowest unstable radial mode (f). The diagram provides an
estimate for the log g value of the observed star, if we assume that the
centers of the observed frequency clusters correspond to the radial mode
frequencies. This assumption is confirmed by examples of well-studied Delta
Scuti variables in which radial modes were definitely identified.
