Equidistant frequency triplets in pulsating stars: the combination mode hypothesis

Context. Multiplet structures are a common feature in pulsating stars, and can be the consequence of rotational splitting, mode interaction or sinusoidal amplitude variations. Aims. In this paper we examine the phenomenon of (nearly) equidistant triplets, which are unlikely to be caused by rotational splitting, in different types of pulsating stars: a delta Scuti variable (1 Mon), an RR Lyrae variable (RR Lyr) and a short-period Cepheid (V473 Lyr). We examine the hypothesis that one of the modes forming the triplet results from a combination of the other two modes. Methods. The analyses were carried out on recent data sets by using multiple-frequency analyses and statistics with the package PERIOD04. In particular, the small departures from equidistance were calculated for the three selected stars. Results. For the d Scuti variable 1 Mon, the departure from equidistance is only 0.000079 +/- 0.000001 cd(-1) (or 0.91 +/- 0.01 nHz). For 1 Mon the Combination Mode Hypothesis with a mode excited by resonance is the most probable explanation. For the star RR Lyr, the hypothesis of resonance through a combination of modes should be considered. The results for the best-studied cepheid with a Blazhko period (V473 Lyr) are inconclusive because of an unfavorable period of 1.49d and insufficient data.