In the so-called Cepheid and Geminid variables of short period, the principal characteristics of the light variation are as follows:—
“1. The light varies without pause.
“2. The amount of their light variation is usually about 1 magnitude.
“3. Their periods are short—a few days only.
“4. They are of a spectral type approximately solar; no Orion, Sirian or Arcturian stars having been found among them.
“5. They seem to be found in greater numbers in certain parts of the sky, notably in the Milky Way, but exhibit no tendency to form clusters.
“6. All those stars whose radial velocities have been studied have been found to be binaries whose period of orbital revolution coincides with that of their light change.
“7. The orbits, so far as determined, are all small, a sin i being 2,000,000 kilometres or less.
“8. Their maximum light synchronizes with their maximum velocity of approach, and minimum light with maximum velocity of recession.
“9. No case has been found in which the spectrum of more than one component has been bright enough to be recorded in the spectrograms.”[347]