A relationship between the radius of a pulsating variable and its luminosity. The radius is derived from the radial-velocity curve, which generally has a different phase from the luminosity curve; the amount of phase lag depends on the specific type of variable. In a Beta Cephei star, for example, the luminosity lags behind the radial-velocity curve by almost exactly one-quarter of the period, so that maximum luminosity corresponds to minimum radius. In Cepheid variables and RR Lyrae stars, by contrast, maximum light corresponds to an intermediate radius at the maximum velocity of expansion.