Period-magnitude diagram for FaRPB stars with km s^-1. Solid vertical lines link periods within each multi-periodic star, with the second and third periods drawn with smaller indicators. The two solid lines locate the PL relation discovered in this Letter from a combined fit to the data on the two sequences. The dashed and dotted lines illustrate relations that would result from analytical arguments linking the PL relation to stellar rotation and Cepheid-like pulsation, respectively (see text). The vsini values are color-coded according to the color scale on the right of the figure.