Determination of the shape of a double star 559 



shows a form of equilibrium of rotating liquid as computed by me 

 some years before, and it was added for the sake of comparison. 



On turning back to Fig. 5 the reader will see in the smooth dotted 

 curve the light variation which would be exhibited by such a binary 

 system as this. The curve is the result of computation and it is 

 impossible not to be struck by the closeness of the coincidence with 

 the series of black dots which denote the observations. 



It is virtually certain that RR Centauri is a case of an eclipsing 

 binary system, and that the two stars are close together. It is not of 

 course proved that the figures of the stars are ellipsoids, but gravita- 

 tion must deform them into a pair of elongated bodies, and, on the 

 assumptions that they are not enveloped in an absorptive atmosphere 

 and that they are ellipsoidal, their shapes must be as shown in the 

 figure. 



This light-curve gives an excellent illustration of what we have 

 reason to believe to be a stage in the evolution of stars, when a single 

 star is proceeding to separate into a binary one. 



As the star is faint, there is as yet no direct spectroscopic evidence 

 of orbital motion. Let us turn therefore to the case of another star, 

 namely V Puppis, in which such evidence does already exist. I give 

 an account of it, because it presents a peculiarly interesting confirma- 

 tion of the correctness of the theory. 



In 1895 Pickering announced in the Harvard Circular No. 14 

 that the spectroscopic observations at Arequipa proved V Puppis 

 to be a double star with a period of 3 d 2 h 46 m . Now when Roberts 

 discussed its light-curve he found that the period was l d 10 h 54 m 27 s , 

 and on account of this serious discrepancy he effected the reduction 

 only on the simple assumption that the two stars were spherical, and 

 thus obtained a fairly good representation of the light-curve. It 

 appeared that the orbit was circular and that the two spheres 

 were not quite in contact. Obviously if the stars had been assumed 

 to be ellipsoids they would have been found to overlap, as was the 

 case for RR Centauri 1 . The matter rested thus for some months 

 until the spectroscopic evidence was re-examined by Miss Cannon 

 on behalf of Professor Pickering, and we find in the notes on 

 p. 177 of Vol. xxvin. of the Annals of the Harvard Observatory 

 the following : "A.G.C. 10534. This star, which is the Algol variable, 

 V Puppis, has been found to be a spectroscopic binary. The 

 period l d '454 (i.e. l d 10 h 54 m ) satisfies the observations of the 

 changes in light, and of the varying separation of the lines of the 

 spectrum. The spectrum has been examined on 61 plates, on 23 

 of which the lines are double." Thus we have valuable evidence 

 in confirmation of the correctness of the conclusions drawn from the 



1 Astrophysical Journ. Vol. xni. (1901), p. 177. 



