There is one type of pulsating variable that lies nowhere near the Cepheid instability strip in the HR diagram: the ? CEPHEI (or ? Canis Majoris) stars. These hot, massive stars have periods of a few hours and ranges of a few hundredths of a magnitude in visible light. The ranges in ultraviolet light are much larger, be-cause most of the energy of these hot stars is emitted in the ultra-violet, and the radial velocity variations in these stars are among the largest in any pulsating variables.

? Cephei stars are notable for several reasons. To begin with, they are the most numerous pulsating variables among the bright stars; they include ? Virginis (Spica), V Crucis and V Centauri, among others. They exhibit several interesting pathological effects which have absorbed the efforts of astronomers for several decades. Yet despite this effort, the nature and cause of the pulsation are completely unknown!

The Cepheid instability strip merges, at its upper end, with a large instability region in the upper right portion of the HR diagram. The transition region is occupied by a group of rare, luminous stars of spectral type F, G and K, called RV TAURI VARIABLES. Their periods are 50 to 100 days and are weakly correlated with the luminosity. Their light curves have a unique feature – alternating shallow and deep minima – whose cause is completely unknown^ They also emit strong infrared radiation, apparently from a shell of material which has been driven off the star by the pulsation.

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