2002MNRAS.335..399H

We present the analysis of 1 week of single-site high-speed CCD photometric observations of the cool ZZ Ceti star HS0507+0434B. 10 independent frequencies are detected in the light variations of the star: one singlet and three nearly equally spaced triplets. We argue that these triplets are caused by rotationally split modes of spherical degree ℓ= 1. This is the first detection of consistent multiplet structure in the amplitude spectrum of a cool ZZ Ceti star and it allows us to determine the rotation period of the star: 1.70 ± 0.11 d.

We report exactly equal frequency, not period, spacings between the detected mode groups. In addition, certain pairs of modes from the four principal groups have frequency ratios that are very close to 3 : 4 or 4 : 5; while these ratios are nearly exact (within one part in 10 ⁴), they still lie outside the computed error bars. We speculate that these relationships between different frequencies could be caused by resonances. One of the three triplets may not be constant in amplitude and/or frequency.

We compare our frequency solution for the combination frequencies (of which we detected 38) to Wu's model thereof. We obtain consistent results when trying to infer the convective thermal time and the inclination angle of the rotational axis of the star. Theoretical combination-frequency amplitude spectra also resemble those of the observations well, and direct theoretical predictions of the observed second-order light-curve distortions were also reasonably successful assuming the three triplets are caused by ℓ= 1 modes. Attempts to reproduce the observed combination frequencies adopting all possible ℓ= 2 identifications for the triplets did not provide similarly consistent results, supporting their identification with ℓ= 1.