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 342" 46". Now when Roberts 
discussed its light-curve he found that the period was 14 10® 54™ 27%, 
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’. 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. xxvitl. 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 14454 (ie. 12 10" 54”) 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. x1. (1901), p. 177. 
