  Multiplet structures are a common feature in pulsating stars, and can be the
consequence of rotational splitting, mode interaction or sinusoidal amplitude
variations. 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 (V743 Lyr). We examine the hypothesis that
one of the modes forming the triplet results from a combination of the other
two modes. 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. For the Delta Scuti variable 1 Mon, the departure from
equidistance is only 0.000079 +- 0.000001 c/d (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 (V743 Lyr) are inconclusive because of an
unfavorable period of 1.49d and insufficient data.
