Below the RR Lyrae variables in the Cepheid instability strip, we find the DWARF CEPHEID VARIABLES (sometimes called AI Velorum stars). As their name implies, they are smaller and fainter than Cepheids (or RR Lyrae stars). They have periods of 0.05 to 0.3 day, and ranges of 0.3 to 0.7 magnitude. Those with the larger ranges can be studied visually through a small telescope; a complete cycle lasts for only a few hours. Several dwarf Cepheids have light curves with slowly rising and falling range (figure 4.14). This phenomenon is called MODULATION, and arises because the star is pulsating simultaneously in two different modes, usually the fundamental and the first harmonic. We do not know whether the dwarf Cepheids are low-mass stars, slightly younger and richer in heavy elements than the RR Lyrae stars, or whether they are normal-mass young stars, closely related to the 6 Scuti variables, which we will describe next.

The Cepheid instability strip intersects the richly-populated main sequence at a temperature corresponding to spectral type FO. Stars on and near the main sequence, with spectral types near FO, would be expected to pulsate – and they do. These are 5 Scuti variables. However, the periods are only an hour or two the ranges are as small as 0.01 magnitude, and the light curves are quite irregular.

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