Close Binary Systems. 113 



In the third light curve, that of CPD— 41° 4511 ^^^^^ }^ ^ 

 double depression, one being very slight. In this case the fainter 

 star is sufficiently bright to manifest a secondary eclipse when the 

 i Tighter companion gets in front of it. The primary eclipse takes 

 place when the faint component obscures the light of the bright one. 

 It may be mentioned here that in the case of CPD — 41° 45 11 the 

 brighter star is six times more lustrous than the fainter star, although 

 it is of the same size. 



In the fourth and fifth light curves we have represented a re- 

 markable class of binary stars. Both X Carinae and RR Centauri 

 exhibit two almost equal eclipses each revolution. The interpreta- 

 tion of this is that these systems are composed of two stars almost 

 equal in size and brightness. 



Thus in the five light curves exhibited in Fig. i we have testi- 

 mony to at least three stages of double star evolution : — 



(i) First, when both stars of a binary system are equal in 

 brightness. This is in all probability the lirst stage. 

 X Carinae and RR Centauri may be taken as types of 

 this sub-division. As a rule these stars circle round one 

 another in a very short period, usually a few hours. 



(2) Second, when one star is distinctly fainter than the other. 



For some reason or another one of the components has 

 cooled down : it has reached the second stage in its 

 development. 



(3) Third, when one of the twain is quite dark. This is the 



third stage. What stages still lie before the chilled mass 

 we cannot tell. It may become a fertile jilanet or a 

 sterile satellite. 



These are the three outstanding facts which we can deduce 

 from the five light curves under consideration. But when we carry 

 our inspection one step further we find important differences between 

 each one of the five curves. 



Thus we observe that the eclipse of S Velorum (ist light curve) 

 remains at its minimum phase for six hours. In the case of R 

 Arae (2nd light curve) there is no halting when the light changes 

 have reached their ebb : flow follows ebb without a halt in the varia- 

 tion of this star. 



The only sufficient explanation of the constant minimum phase 

 of S Velorum is this, that the dark companion of this binarv svstem 

 is many times larger than its bright companion. This seems such 

 a strange reversion of the natural order of things — a small central 

 sun and a large dark satellite that at first sight one is inclined to set 

 this explanation aside. Yet no other has been offered. And I 

 take it the explanation is a rational one. as I hope to indicate later 

 on. 



It will be observed also that the light curves of X Carinae or 

 RR Centauri, Avhile alike in essential feature, are different in one or 

 two not unimportant particulars. Thus the light curve of RR 

 Centauri has no constant phase at all, however short. Its light curve 



